agent:  Latin agere, a verb of great semantic breadth (‘drive, lead, act, do’), has been a prolific source of English words. Its past participle, āctus, produced act, action, active, actor, actual, cachet, and exact, while other parts of its paradigm lie behind agile, agitate, ambiguous, coagulate, cogent, cogitate, examine, exigent, exiguous, and prodigal.
Its most obvious offspring, however, are agent (literally ‘(person) doing something’) and agency, formed from the Latin present participial stem agent-. Agere itself is of considerable antiquity, being related to other Indo-European verbs such as Greek ágein ‘drive, lead’, Old Norse aka ‘travel in a vehicle’, and Sanskrit ájati ‘drives’. => act, agile, ambiguous, cachet, cogent, demagogue, exact, examine, prodigal
late 15c., "one who acts," from Latin agentem (nominative agens) "effective, powerful," present participle of agere "to set in motion, drive, lead, conduct" (see act (n.)). Meaning "any natural force or substance which produces a phenomenon" is from 1550s. Meaning "deputy, representative" is from 1590s. Sense of "spy, secret agent" is attested by 1916.