- A posteriori weighting
- A priori weighting
- abscissa
- abundance
- Accuracy
- acinus
- acrocentric
- aculeate
- acuminate
- acute
- adaptation
- adaptativity
- Adaptive zones
- Additive
- adult
- Aepyornis
- agricultural
- akanga
- alarm call
- allantois
- allele
- Allochthonous
- allogrooming
- Allometry
- allopatric
- Allopatric speciation
- Allopolyploidy
- allozyme
- Ambatovaky
- Amboasary-Sud
- Ambohijanahary
- Ambohitantely
- Amino acid
- amnion
- amniote egg
- amorphous
- ampansakabe
- Analabe
- Analamaitso
- Analamazaotra
- Analamera
- Analogy
- anatomy
- ancestor worship
- andevo
- Andohahela
- Andranomena
- Andriamanitra
- Andriana
- Andriana-mpoini-merina
- Andringitra
- angulate
- anisogamy
- Anjanaharibe-Sud
- Ankarafantsika
- Ankarana
- Ankaratra
- ankoma
- annuli
- ANOVA
- Antaifasy
- Antaimoro
- Antaisaka
- Antambahoaka
- Antananarivo
- Antandroy
- Antankarana
- Antanosy
- Antatsimo
- anterior
- Antevaratra
- anus
- apex
- apivorous
- apocrine gland
- Apomorphy
- appendage
- arable
- arachnoid
- arboreal
- arcuate
- Area cladogram
- Area of endemism
- areola
- argentaffine cells
- arid
- aristidia
- articulation
- association
- Assumption 0
- Assumption 1
- Assumption 2
- attenuate
- attenuated
- Attribute
- Autapomorphy
- Autopolyploidy
- autosome
- Availability