a-, an-

(Greek: prefix; 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.

acoprosis (s) (noun), acoproses (pl)
Absence or deficiency of fecal matter in the intestines: Geoffrey was diagnosed with a problem of acoprosis which affected his bowel movements.
acoprous (adjective) (not comparable)
A reference to a person who has no fecal matter in the bowels: Leland was referred to a specialist to determine what was causing the acoprous condition of his intestine.
acorea (s) (noun), acoreas (pl)
Absence of the pupil of the eye: The congenital acorea of Wilbert's left eye pupil could not be corrected by surgery.
acotyledon (s) (noun), acotyledons (pl)
Any plant without seed leaves: The new plant in the garden was just beginning to sprout and had not yet developed acotyledons or new leaves.
acrania (s) (noun), acranias (pl)
A partial or complete absence of the skull: The hikers discovered the skeleton of a fox that appeared to be an acrania because there was no evidence of any head bones.
acroagnosis (s) (noun), acroagnoses (pl)
A decrease of any sensory recognitions or feelings of a limb (arms or legs) of the body: In physiotherapy it became apparent that Clark was experiencing acroagnosis because he was unable to feel any sensations in his arms.
adactyly (s) (noun), adactylies (pl)
The absence of digits on a hand, or both hands; or on a foot, or the feet: The new prosthesis helped Bryant to overcome the adactyly of his left hand.
adermia (s) (noun), adermias (pl)
Having no skin or being without skin: The adermia of the laboratory fish made it impossible for it to survive.

An adermia is defined as an acquired absence of the skin or a specified area of skin.

adiabatic (adjective), more adiabatic, most adiabatic
In thermodynamics, describing a process in which there is no transfer of heat into or out of the system in question: The research indicated that there was an adiabatic loss or gain of heat for the proposed manufacturing of the material.
adiadochocinesia, adiadochocinesis; adiadochokinesia, adiadochokinesis (s) (noun); adiadochocinesias, adiadochocineses; adiadochokinesias, adiadochokineses (pl)
1. Loss of the power to perform rapid alternating movements: This symptom of adiadochocinesia is indicative of a disorder of the cerebellum or its tracts.

Adiadochokinesis involves the inability to stop one movement and then to follow it immediately with another motion in the opposite direction.

2. The inability to perform rapid alternating movements of one or more of the body's extremities: Adiadochocinesis is sometimes requested by physicians during physical examinations so they can determine if the patient has any neurological problems.
3. Etymology: a highly contrived word that is composed of the Greek a-, "without" + diadochos, "successive" + kinesis, "motion".

So, adiadochokinesis is a neurological sign of the inability to perform rapid alternating movements; such as, pronation (turning the hand so the palm faces downward or backward) and supination (having the palm of the hand facing upward).

—Primarily compiled from information located in
Medical Meanings, A Glossary of Word Origins, 2nd edition;
by William S. Haubrich, MD; American College of Physicians;
Philadelphia, Pennsylvania; 2003; page 5.
adiaphoria (s) (noun), adiaphorias (pl)
Non responses to stimuli as a result of some previous exposure to similar reactions: Able's adiaphoria was diagnosed as a failure to respond to physical forces that the doctor applied during his medical examination.
adiathermancy (s) (noun), adiathermancies (pl)
The condition of being impervious to or unaffected by heat waves: Jake's aunt often experienced adiathermancy and so she didn't find the heat of the desert all that uncomfortable.
adiathermanous (adjective), more adiathermanous, most adiathermanous
In physics, a reference to the inability to transmit radiant heat or not affected by heat: The new adiathermanous material was a great advantage for the plumbing and heating industry.
adiathermic (adjective), more adiathermic, most adiathermic
Characteristic of something that is not capable of transmitting radiant heat: The plumber installed adiathermic pipes in the basement of the building.
adipsia (s) (noun), adipsias (pl)
1. The abnormal absence of thirst: Adrian was told that his lack of a desire to drink fluids, even when his body was low on water was diagnosed as adipsia which was resulting in an excess amount of salt in his body.
2. An abnormal avoidance of drinking: As people age, they have a tendency to have adipsia and so they are less likely to notice their thirst and may not drink sufficient fluids when they are needed.