h (1) hour; (2) height.
h Planck's constant.
¹H Protium. See: hydrogen.
²H Deuterium. See: hydrogen.
³H Tritium. See: hydrogen.
HA Hemagglutination assay.
habitat /HAB-ə-tat/ n. The sort of environment in which an organism normally lives and grows.
haematocolpos See: hematocolpos.
haematopoiesis See: hematopoiesis.
hair cell A type of motion-detecting cell found in many different types of animals.
Haldane's Rule A rule, attributed to J.B.S. Haldane, usually stated as: "In a hybrid cross, if one sex is absent, rare or sterile, it will be the heterogametic sex." In mammals this is the male sex, in birds, the female.
hallucinogen /hə-LOOSE-ə-nə-jehn/ n. A drug that induces hallucinations.
hand NAMES OF HAND BONES
haploid /HAP-loid/ adj. (1) in the case of a single-celled eukaryotic organism, having a single complete set of chromosomes; (2) in the case of a multicellular eukaryotic organism, having a single complete set of chromosomes in each somatic cell. See also: diploid.
HDL High-density lipoprotein.
heart A muscular organ, present in animals, that pumps the blood through the circulatory system.
hectare /HECK-tare/ n. A metric unit of area equal to 10,000 square meters.
hectoliter /HECK-tə-leet-er/ n. A metric unit of volume equal to 100 liters.
heliotrope /HEEL-ee-ə-trope, -yə-trope/ n. An organism affected by heliotropism.
heliotropism /hee-lee-AWT-rə-piz-əm/ n. Phototropism in which sunlight serves as the stimulus.
hemangiosarcoma /heem-ANJ-ee-ō-sar-KŌ-mə/ n. A sarcoma arising in the cells that line the blood vessels.
hematocolpos /heem-ə-tō-KOLP-pŎs/ (British: haematocolpos) A blockage of the vagina by an imperforate hymen resulting in retention of menstrual blood.
hematophagous /heem-ə-TAW-fə-gəs/ adj. Feeding on blood.
hematopoiesis /heh-məd-ō-poy-EE-səs/ (Brit haematopoiesis) Alternate spellings: hemopoiesis and (Brit) haemopoiesis n. The formation of the cellular components of blood within the living body. All cellular components of blood arise ultimately from haematopoietic stem cells. This word is often misspelled as "hemopoesis" or "haemopoesis." MORE INFORMATION | CHART SHOWING STEPS IN BLOOD CELL PRODUCTION (HEMATOPOIESIS)
Hemimetabola /hem-ee-mə-TAB-ə-lə/ n. The insect superorder in which the larva fairly closely resembles the adult insect throughout its entire period of development. There are several molts and during the later stages (instars), wings appear. During the last molt, the fully developed adult (imago) emerges. See: holometabolous.
hemiparasitic /hem-ee-par-ə-SIT-ick/ n. A parasitic plant capable of producing some of its nutrients via photosynthesis. PICTURE OF A HEMIPARASITE (Phoradendron serotinum)
hemizygote /hem-ee-ZĪ-gōt, -gut/ n. An individual hemizygous for a gene.
hemoclasia (also hemoclasis) /heem-ō-CLAY-zhə/ n. Disintegration of erythrocytes.
hemoglobin /HEEM-ə-glō-bən/ n. An iron-bearing protein that reversibly binds oxygen; found in red blood cells.
hemoptysis /hee-MAWP-tə-səs/ n. Bloody discharge expelled from mouth or nose; derived from hemorrhage of lungs, trachea, or larynx.
hemorrhage /HEM-ə-rəj/ n. Any abnormal discharge from the blood vessels, whether internal or external.
hemostat /HEM-ə-stat/ n. A medicine or surgical tool used to stop blood flow.
hepatic /hə-PAT-ik/ adj. Of, pertaining to, or arising from the liver.
hepatic flexure /hə-PAT-ik/ n. An angular bend in the large intestine between the ascending and transverse colons.
hepatitis /hep-ə-TIGHT-əs/ n. Inflammation of the liver.
hepatomalacia /HEP-ə-tŎ-mə-LAY-zhə/ n. Softening of the liver.
hepatomegaly /hep-ə-tŎ-MEG-ə-lee/ n. Enlargement or swelling of the liver.
hepatorrhexis /hep-ə-tŎ-RECKS-əs/ n. Rupture of the liver.
herbivore /(H)ERB-bə-vore/ n. Any animal that eat plants.
herbivorous /(h)er-BIV-er-əs/ adj. Of, pertaining to, or being a herbivore.
heredity /her-RED-ə-tee/ n. The transmission of traits from one generation to another by genetic means. — hereditary /hə-RED-ə-tare-ee/
heritable /HARE-id-ə-bəl, -əd-ə-bəl/ adj. Capable of transmitted from one generation to another by genetic means.
hermaphrodite /her-MAWF-rə-dite/ n. An individual with both male and female organs. MORE INFORMATION
hermaphroditic /her-MAWF-rə-DIT-ik/ adj. Having the quality of, or being a hermaphrodite.
herpetic /her-PET-ik/ adj. Of or pertaining to herpes.
heterochromatin /HEHD-ə-rō-KRōM-ə-tin/ n. Chromatin that is condensed (visible with a light microscope) during interphase. Heterochromatin is late replicating, transcriptionally inactive, and is composed mostly of repetitive DNA.
heterochronia /HEHD-ə-rŎ-KRō-nee-ə/ n. A difference in timing during the course of development; sometimes proposed as a mechanism of evolutionary change.
heterocysts /HEHD-ə-rə-SISTS/ n. Cyanobacterial cells that carry out nitrogen fixation.
heterodromous /HEHD-ə-RAWD-drum-əs/ adj. Having branches that spiral in the opposite direction to the spiraling of the main stem.
heterogametic sex /HEHD-ə-rō-gə-MEE-tick/ n. The sex that produces two types of sex cells.
heterosis /HEHD-ə-RŌ-səs/ n. In a hybrid, traits falling outside the range of parental variation. For example, a hybrid might be larger (or smaller) than either of its parents — heterotic /HEHD-ə-RAWT-ick/ MORE INFORMATION
heterosporous /HEHD-ə-rə-SPORE-əs/ adj. Pertaining to plants having a sporophyte that produces two types of gametophytes of different sexes.
heterotaxis (also heterotaxy) /HEHD-er-ō-TAX-əs/ n. Abnormal arrangement of parts.
heterotroph /HEHD-ə-rə-TROFE, -TRAWF/ n. An organism that cannot produce from an inorganic substrate all of the organic molecules it requires; it instead obtains organic molecules by eating other organisms or substances produced by them; used in opposition to autotroph.
heterozygote /HEHD-er-ō-ZĪ-gōt/ n. A heterozygous individual.
hexacanth /HEX-ə-KAHNTH/ adj. Having six hooks, thorns, or barbs.
hexaploid /HEX-ə-ploid/ (1) adj. exhibiting hexaploidy; (2) n. an organism, cell, or type exhibiting hexaploidy.
HGPRT n. Hypoxanthine-guanine phosphoribosyltransferase.
hibernation /HĪ-burn-NAY-shən/ n. A physiological state, characterized by reduced metabolic activity, that allows an animal to survive cold seasons with little or no food.
hidradenoma /hī-drad-ə-NŌ-mə/ n. Adenoma of the sweat glands.
hidroschesis /hī-DRAWS-keh-səs/ n. Non-secretion of sweat.
hilum (pl hila) /HĪ-ləm/ n. The mark left on a seed by the former attachment of the ovule stalk.
histamine /HIST-ə-meen/ n. A substance released by damaged cells; causes blood vessel dilation which results in the red flush characteristic of injured tissue.
histology /hist-TAWL-ə-jee/ n. The study of the microscopic structure of cellular tissue — histologist /hist-TAWL-ə-jist/
histotome /HIST-ə-tome/ n. An instrument for preparing thin slices of tissue. Also: microtome
histotomy /hiss-TAWT-tə-mee/ n. (1) the cutting of thin tissue sections for microscopic study; (2) dissection of tissue.
HIV Human immunodeficiency virus.
HMG-CoA n. b-hydroxy-b-methylglutaryl-CoA.
hnRNA Heterogeneous nuclear RNA.
holdfast n. An anchoring process of an organism.
holarctic /hōl-ARK-tick, hawl-/ adj. (1) capitalized: the northern regions of both the eastern and western hemispheres; (2) not capitalized: of or pertaining to the Holarctic.
Holocene /HŎL-ə-seen, HAWL-/ n. The most recent geological epoch, which began at the end of the last ice age (~12,000 BP).
Holometabola /Hō-lō-mə-TAB-ə-lə/ n. See: holometabolous.
holometabolous /hō-lō-mə-TAB-ə-ləs/ adj. Belonging to the insect superorder Holometabola; composed of all insects that undergo a complete metamorphosis; a larva bears no resemblance whatever to the fully developed adult insect. During development, an individual molts repeatedly, and eventually pupates. During the pupal stage the original larval organs are destroyed and the organs of the adult stage (imago) develop. The imago then emerges from its chrysalis or cocoon. See: Hemimetabola, hemimetabolous.
homeobox /HōM-ee-ō-box, -ə-box/ n. A conserved sequence found within the homeotic genes of Drosophila; also found in embryonically expressed genes of mammals and amphibians. Homeoboxs are about 180 base pairs long and codes for a protein that can bind DNA.
homeostasis /hŎm-ee-ō-STAY-sis/ n. A tendency toward the maintenance of a steady physiological state.
homeotic gene /hŎm-ee-AWT-ik/ n. A gene that controls overall development (morphogenesis).
hominids /HAW-mə-nidz, -nədz/ n. (1) recent humans and extinct ancestral and related forms — this is the sense of the word used in the articles on human evolution appearing on this website (in recent literature one school of thought says hominid should be replaced by hominin); See: Hominidae; (2) members of the family Hominidae. See: hominin, Hominidae. MORE INFORMATION | HUMAN EVOLUTION TIMELINE
Hominidae /haw-MIN-ə-dee/ n. A family of the mammalian order Primates. Some scientists use a definition of Hominidae that includes not only humans, but also the great apes (chimpanzees, bonobos, orangutans, and gorillas), and instead use the term hominins to refer to what are called "hominids" on this website. (The latter is the traditional term for recent humans and extinct ancestral and related forms. The former is a new term used scientists who think names should be changed as theories of relationship change.)
hominine /HAW-mə-neen/ n. A member of the subfamily Homininae.
hominins /HAW-mə-ninz/ n. (1) extinct ancestral and related forms (this term is used with this meaning by some scientists. Others say hominid to mean the same thing); (2) a member of the tribe Hominini.
hominoid /HAW-mə-noid/ n. Any member of the superfamily containing all apes and hominids, the Hominoidea.
homodromous /HōM-ə-DROME-əs/ adj. Having branches that spiral in the same direction as the spiraling of the main stem.
homogametic sex /HōM-mō-gə-MEET-ik/ n. The sex producing a single type of gamete.
homologous chromosomes /hə-MALL-ə-gəs/ n. See: homologs.
homologs /HŌM-ə-lawgs/ (also homologous chromosomes /hə-MALL-ə-gəs/) (1) chromosomes having the same loci, in the same order (a diploid cell has two copies of each homolog, one derived from each parent), also called homologous chromosomes; a "chromosome pair" is a pair of homologs (2) genes having the same origin.
homology /hə-MAWL-ə-jee/ n. (1) of chromosomes, or regions of chromosomes: having the same loci, in the same order; (2) any similarity between organisms considered to reflect relationship (note: the term homology is often used loosely to refer to what should be more properly called similarity between DNA or protein sequences); See: homologous traits. See: convergence MORE INFORMATION
homotaxis (also homotaxy) /HŌM-ō-TACKS-əs, HOAM-ə-/ n. Similarity of arrangement.
homotypy /HŌ-mə-TIP-ee/ n. Condition of being of the same basic type.
homozygote /HŌ-mō-ZĪ-gōt or HŌ-mə-ZĪ-gət/ n. A homozygous individual.
hormones /HORE-moans/ n. Circulating molecules that serve as signals for particular body processes to occur by interacting with target cells.
HPLC High-pressure (or high-performance) liquid chromatography.
humor /HYOOM-er/ n. Any fluid serving a function within the body.
humoral /HYOOM-er-əl/ adj. Pertaining to the various fluids functioning within the body, and their contents.
humoral immune system /HYOOM-er-əl/ n. The portion of the immune system that produces antibodies that circulate in the blood and lymph.
hyaline /HĪ-ə-lin/ adj. Glassy, transparent.
hyaloiditis /hī-ə-loi-DĪT-əs/ n. Inflammation of the hyaloid membrane.
hyaloid membrane /HĪ-ə-loid/ n. The transparent membrane enclosing the vitreous membrane.
hybrid /HĪ-brid, -brəd/ n. (1) the offspring of genetically distinct parents; (2) the offspring of mating between individuals treated as separate species MORE INFORMATION; (3) a single strand of DNA annealed to a strand of complementary RNA; (4) a virus composed of units derived from two or more parental viruses. GLOSSARY OF TERMINOLOGY RELATING TO HYBRIDS
hybrid inviability /HĪ-brid, -brəd/ n. Impairment of a hybrid's ability to survive (in comparison with its parents). MORE INFORMATION
hybridization /HĪ-brid-ī-ZAY-shən, -brəd-/ n. (1) interbreeding between distinct genetic types, including types treated as distinct species; (2) the process of joining two complementary strands of DNA, or one each of DNA and RNA, to form a double-stranded molecule.
hybridoma /HĪ-brid-Ŏ-mə, -brəd-/ n. A type of cell used in the artificial production of monoclonal antibodies. It is a hybrid of two types of cells, a normal antibody-producing lymphocyte and a myeloma cell. The hybrid cell has the capability of indefinite cell growth, but also produces the desired antibodies.
hydrocephalic /HĪ-drə-seff-AL-ick, HĪ-drə-/ (also hydrocephaloid /-SEFF-ə-loid/) n. Of or relating to hydrocephalus.
hydrocephalus /HĪ-drō-SEFF-ə-ləs, HĪ-drə-/ n. Abnormal retention of cerebrospinal fluid in the brain; results from blockage of normal circulation of the fluid. In children affected by this condition, the cranium is enlarged and thin.
hydrogel /HĪ-drō-jehl, HĪ-drə-/ n. A gel composed of water contained within a matrix of hydrophilic polymers. The water content of such gels is often greater then 99%, which gives them characteristics similar to those of living tissues.
hydrogen (H) /HĪ-drə-jən, -jen/ n. The most abundant chemical element, a colorless, odorless, tasteless, flammable gas with a single valence electron; atomic number 1, atomic weight 1.00794 g•mol⁻¹; hydrogen is the most abundant element in the universe, comprising 75% of its mass, but rare as a free element on Earth. Even in its compounded forms it comprises only about 1% of terrestrial matter. It derives its name from the fact that it produces water when burned. Hydrogen has three isotopes, protium (¹H), deuterium (²H), and tritium (³H); 99.985% exists as the isotope ¹H. It is an essential ingredient of life since it is a component of both water and all hydrocarbons.
hydrogen bond /HĪ-drə-jən, -jen/ n. A weak bond formed by the attraction of a partially positive hydrogen atom (in a polar covalent bond) to a partially negative atom in a polar covalent bond of some other molecule.
hydrogen ion concentration (pH) /HĪ-drə-jən, -jen/ n. A measure of the number of hydrogen ions present in a solution. It is measured in terms of the logarithm of the reciprocal of the concentration of hydrogen ions in grams per liter of solution, which yields a scale of 0 to 14. A solution with a pH below 7 is acidic, with one above, basic. Note: a lower pH occurs at a higher hydrogen ion concentration.
hydrophobic /HĪ-drə-Fō-bik/ adj. Repelled by water; used in reference to molecules, or portions of molecules (e.g., alkyl groups), that are relatively insoluble in water. Water-repelling biological membranes are composed of hydrophobic molecules.
hydroxide /hī-DRAWK-xīd/ n. A chemical compound containing a hydroxyl group.
hydroxyl group /hī-DRAWK-səl/ n. The univalent hydrophilic radical OH, which commonly combines with metal ions or other radicals acting as metals (for example, NH₄), to form hydroxides.
hygroscopic /hī-grə-SCAW-pik/ adj. (1) in chemistry, having the tendency to absorb water from the environment; (2) in botany, sensitive to moisture.
Hymenoptera /hī-mən-NAWP-ter-ə/ n. The insect order that includes bees, ants, and wasps.
hyoepiglottic Pertaining to the hyoid bone and the epiglottis.
hypercalcemia /HĪ-per-kal-SEEM-ee-yə/ (British: hypercalcaemia) n. Excessive calcium in the blood.
hyperglycemia /HĪ-per-glī-SEEM-ee-yə/ (British: hyperglycaemia) n. Excessive glucose in the blood.
hyperostosis /HĪ-per-aw-STŌ-səs/ n. Excessive, abnormal bone growth.
hyperphoria /HĪ-per-FORE-ee-yə/ n. A condition in which one eye tends to turn upward.
hypertension /HĪ-per-TEN-shən/ n. Increased blood pressure.
hypocellular /hī-pə-CELL-yə-ler/ adj. Containing fewer cells than under normal circumstances.
hypochondriac /hī-pə-KAWN-dree-ak/ n. A person affected by an abnormally heightened and unreasonable fear of disease.
hypochromia /hī-pə-KROME-ee-yə/ n. (1) lack of, or a reduced degree of, coloration; (2) deficiency of hemoglobin.
hypogeal /hī-pə-JEE-əl/ adj. Developing or growing beneath the soil.
hypoplasia /hī-pə-PLAY-zhə/ n. A condition of arrested development in which an organ or part remains below the normal size or in an immature state.
hypotonic solution /hī-pə-TAWN-ik/ n. See: hypertonic solution.
hystera /HIST-er-rə/ n. The uterus.
hysterectomy /hist-er-RECT-ə-mee/ n. Surgical removal of the uterus.
hysterorrhexis /his-ter-ō-RECK-səs/ n. Rupture of the uterus.
hysterotrismus /his-ter-o-TRIS-məs/ n. A spasm of the uterus.
Hystrix /HISS-tricks/ n. A genus of Old World porcupines.
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