a-, an- +

(Greek: a prefix meaning: no, absence of, without, lack of, not)

These prefixes are normally used with elements of Greek origin, a- is used before consonants and an- is used before vowels.

It affects the meanings of hundreds of words.

There are too many words that use these prefix elements to list all of them on this site; however, there are significant examples listed in this and the other units where they exist.

aneuthanasia

Painful death.

anhedonia

1. Absence of pleasure from the performance of acts that would ordinarily be pleasurable.
2. The inability to feel pleasure.

anhidrosis, anidrosis

1. Absence of sweat.
2. Not sweating.
3. The deficiency or absence of perspiration.
4. Absence of sweating in horses, a condition more noticeable in hot climates; affected animals develop severe dyspnea and cannot work.
5. Absence or deficiency of sweating; in humans this is usually due to absence or paralysis of the sweat glands or to some kind of obstruction of the sweat ducts.

For some people, the inability to sweat may seem a blessing; but it is not, since to sweat makes it possible for us to stay cooler. Anhidrosis creates a dangerous inability to tolerate heat.

anhydrase

An enzyme that catalyzes the removal of water from a compound; most such enzymes are now known as hydrases, hydrolyases, or dehydratases.

anhydration

A lack of water in the system.

anhydride

A compound derived by the removal of water from an acid or other compound.

anhydrobiosis

1. Dormancy induced by low humidity or by drying out.
2. A state caused by dehydration, in which an organism's metabolism is reduced to an imperceptible level.

anhydrous

Deprived or destitute of water; without water.

aniconia

Absence of mental imagery.

aniso

A word root meaning “not equal”.

anisothenic

Having unequal strength.

anisotropic

1. Having unequal responses to external stimuli.
2. Having properties that vary depending on the various directions of measurements.
3. Not having properties that are the same in all directions.
4. Showing different properties as to velocity of light transmission, conductivity of heat or electricity, compressibility, and so on, in different directions.

anodontia

A congenital absence of the teeth other than by extraction or impaction.

anodyne

A medicine or remedy that soothes or relieves pain.

anoesis, anoesia

Absence of cognition or knowledge; a state of sheer feeling, having no reference to objects; noncognitive consciousness.