| Definition -
A sentence in which the subject is the agent of the verb's action.
Note: As opposed to the passive voice, where the subject is the verb's object.
(1) Mike ate the microphone
(This is in the active voice because the subject Mike is the verb's agent)
(2) Mike was eaten by the microphone
(This is in the passive voice because the subject Mike is the object of the verb eaten)
Oxford English Dictionary - Its first citation is from 1530:
"The thyrde parsonnes plurelles of verbes actyves in the frenche tonge … ende in ent."
(Jehan Palsgrave, Lesclarcissement de la langue françoyse, 4)