| Term | Definition |
| acrimonious | harsh in speech or behavior |
| acronym | word formed from initials |
| acrophobia | fear of heights |
| actuate | to put into motion or action |
| acumen | mental keeness |
| acute | keen |
| ad infinitum | forever |
| ab lib | to act or speak without preparation |
| adage | a familiar saying |
| adamant | stubborn |
| adapt | to adjust |
| addendum | something added as a supplement |
| addled | confused |
| adduce | to give an example in proving something |
| adept | highly skilled |
| adherent | follower or supporter |
| adipose | fatty |
| adjacent | adjoining |
| adjudicate | to judge |
| adjunct | a subordinate |