Environmental Studies
- Aridisol
- aridity index
- aristogenesis
- arkose
- arousal
- arrhenotoky
- arroyo
- artifact
- artificial classification
- artificial freezing
- Artificial recharge
- artificial selection
- Ascencion and St Helena floral region
- assemblage zone
- assimilate
- Assimilation
- association analysis
- association measure
- assortative mating
- asymmetric valley