- antigen
- antihistamines
- antilog
- antimycotic
- antinomy
- antipole
- antiquary
- antiquated
- antiquity
- aorta
- apart
- ape
- apex
- apostate
- apostatize
- apothecary
- apparel
- appear
- applicant
- application
- application form
- application software
- application window
- applies
- appointed time
- apprehend
- apprehension
- apprenticeship
- apprize
- approbate
- appropriate
- aptitude test
- aquifer
- aquiline
- arabia
- arabian sea
- arable
- archaeology
- archaic
- archbishop
- archbishopric
- archdeacon
- archer
- archetypal
- architectonic
- architectural
- architecture
- archive
- areal flood
- areca
- areola
- argon
- argosy
- argot
- arguably
- argument
- armadillo
- armored
- armorer
- armour
- arpeggio
- arrangement
- arrest
- arrestable
- articulate
- articulated
- artificial insemination
- azolla
- as helpless as a babe
- as if
- ascetic
- asphodel
- asr
- assessee
- assign
- assignation
- astral
- astray
- astringent
- astrolabe
- astrological scholar
- astronomical
- astronomy
- astrophysics
- azure
- at a loss
- at a pinch
- at a premium
- at a stroke
- at first glance
- at hand
- at home
- at last
- at loggerheads
- at the eleventh hour
- at the helm
- at the mercy of
- at your leisure
- atavism
- atomy