Common Words:
* action
* active
* actively
* activity
* actor
* actress
* actual
* actually
Less Common Words:
* activate
* activation
* activist
* acutely
* actuality
* actuary
* actinic
* actinium
* actionable
Words in Specific Fields:
* acting (theater/film)
* acupuncture (medicine)
* acrobatics (performance art)
Let me know if you'd like to see any more words!