Encyclopedias, Almanacs, Transcripts, and Maps
- Acipenseriformes (Sturgeons and Paddlefishes)
- Acoela (Acoels)
- Acosta, Gabriel
- act of Congress
- act of Parliament
- Activated Sludge
- Actors Studio
- Acute Effects
- ACV
- Adad
- Adams, Ansel Easton (1902 – 1984) American Photographer and Conservationist
- Adams, Hannah
- Adaptations for Aquatic Life
- Adaptations for Flight
- Adaptations for Subterranean Life
- Adaptive Management
- addition reaction
- Adenophorea (Roundworms)
- adjutant stork
- Admirals Cup