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H, h

haustus (a draft of medicine); height; hemagglutinin; henry; hora; hour; horizontal; hypermetropia.


Symbol for the element hydrogen.


Symbol for protium.


Symbol for deuterium.


Symbol for tritium, a radioactive isotope of hydrogen.


Symbol for hydrogen ion.


Symbol for hecto, a term used in SI units.


Symbol for Planck's constant (the amount of energy in a photon). The constant, ħ, = 6.626068×10-34 m2 kg/s.

[H+], cH+

Symbol for molar hydrogen ion concentration.




hypoglycemia-associated autonomic failure.


hepatitis A antigen.


Highly active antiretroviral therapy.

HAART attack

A colloquial term for IRIS (the immune reconstitution inflammatory syndrome).


(hă-bē′nă, hă-bē′nē″, hă-bē′nī″) pl. habenae [L. habena, rein] 1. A frenum. SEE: frenum. 2. A habenula. SEE: habenula.


(hă-ben′yŭ-lă, hă-ben′yŭ-lē″, hă-ben′yŭ-lī″) pl. habenulae [L. habenula, little rein, strap] 1. A frenum or any reinlike or whiplike structure. 2. A peduncle or stalk attached to the pineal body of the brain. Fibers that travel posteriorly along the dorsomedial border of the thalamus to the habenular ganglia (epithalamus) resemble reins. 3. A narrow bandlike stricture. habenular (hă-ben′yŭ-lăr), adj.

Haber-Weiss reaction

(hob′ĕr-vīs′) [Fritz Haber, Ger. physical chemist, 1868–1934; Joseph Weiss, Haber's student] The generation of toxic oxygen and hydroxyl radicals from hydrogen peroxide and superoxide. These radicals contribute to cell injury in many diseases, e.g., in the brain or heart after a stroke or heart attack.


(hă-bĭl″ĭ-tā′shŭn) 1. The process of educating or training persons with functional limitation to improve their ability to function in society. 2. Qualification for office. 3. Academic accreditation. SEE: rehabilitation.


[L. habitus, state, style] 1. A motor pattern executed with facility following constant or frequent repetition; an act performed at first in a voluntary manner but after sufficient repetition as a reflex action. Habits result from the passing of impulses through a particular set of neurons and synapses many times. 2. A particular type of dress or garb. 3. Mental or moral constitution or disposition. 4. Bodily appearance ...

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