| Term | Definition |
| a | not; without |
| amoral | not moral; without a sense of moral responsibility |
| amorphous | without (having no) definite form or shape |
| anarchy | total absence of rule or government; confusion; disorder |
| anemia | lack of a normal number of red blood cells |
| anesthesia | loss of feeling or sensation resulting from either, chloroform, novocaine, etc. |
| anesthetic | drug that produces anesthesia |
| anhydrous | destitute of (without) water |
| anomaly | deviation from the common rule |
| anomalous | not normal; abnormal |
| anonymous | nameless; of unknown or unnamed origin |
| anoxia | deprivation of (state of being without) oxygen |
| apnea | temporary cessation of breathing |
| aseptic | free from disease causing microorganisms |
| asymptomatic | showing symptoms of disease |
| atheism | godlessness; denial of the existence of a supreme being |
| atrophy | lack of growth from disuse or want of nourishment |
| atypical | unlike the typical |