Merriam-Webster's Collegiate Dictionary:
activation
activation analysis
activation energy
activator
active
active immunity
active site
active transport
active-matrix
actively
activeness
activewear
activism
activist
activistic
activity
actomyosin
acton
actor
actorish
actorly
actress
actressy
acts
acts of the apostles
actual
actual cash value
actuality
actualization
actualize
actually
actuarial
actuarially
actuary
actuate
actuation
actuator
acuity
aculeate
acumen
acuminate
acupressure
acupuncture
acupuncturist
acute
acutely
acuteness
acv
acyclic
acyclovir
acyl
acylate
acylation
ad
ad arbitrium
ad astra per aspera
ad damm?m
ad eundem
ad eundem gradum
ad extremum
ad feminam
ad hoc
ad hominem
ad infinitum
ad int
ad interim
ad kalendas graecas
ad lib
ad libitum
ad loc
ad majorem dei gloriam
ad nauseam
ad patres
ad referendum
ad rem
ad unguem
ad utrumque paratus
ad val
ad valorem
ad verbum
ad vivum
ad-
ad-lib
ad?lie
ad?lie penguin
ada
adage
adagio
adak
adalia
adam
adam's apple
adam's bridge
adam's needle
adam's peak
adam-and-eve
adamance
