| Acrocephalus melanopogon | Moustached Warbler |
| Acrocephalus orinus | Large-billed Reed-warbler |
| Acrochordus | Acrochordidae |
| Acrocoro Etiopico | Ethiopian Highlands |
| acrofobia | fear of heights |
| acromatico | achromatic |
| acromegalia | acromegaly |
| acronimo | acronym |
| Acronimo ridondante | RAS syndrome |
| acropoli | acropolis |
| Acropoli di Atene | Acropolis of Athens |
| Across the Universe | Across the Universe |
| acrostico | acrostic |
| Acroterio | Acroterion |
| Acrux | Alpha Crucis |
| Acta Apostolicae Sedis | Acta Apostolicae Sedis |
| Actea | Claudia Acte |
| actina | actin |
| Actiniaria | Sea anemone |
| Actinidia chinensis | Kiwi |
| Actinobacteria | Actinobacteria |
| actinomicosi | actinomycosis |
| Actinomyces | Actinomyces |
| Actinopterygii | Actinopterygii |
| Action Directe | Action directe |
| Action Française | Action Française |
| Action Learning | Action learning |
| Action Replay | Datel Action Replay |
| Action RPG | Action role-playing game |
| Actions per minute | Actions per minute |
| ActionScript | ActionScript |
| Actitis hypoleucos | Common Sandpiper |
| Active Cavity Radiometer Irradiance Monitor Satellite | Active Cavity Radiometer Irradiance Monitor Satellite |
| Active Directory | Active directory |
| Active Server Pages | Active Server Pages |
| Active time battle | Final Fantasy battle systems |
| Active Virus Shield | AOL Active Virus Shield |
| ActiveX | Component Object Model |
| Activity Based Costing | Activity-based costing |
| ActRaiser | ActRaiser |
| Acuecucyoticihuati | Chalchiuhtlicue |
| acufene | tinnitus |
| acuire | sharpen |
