1590s, "putting forth activity, active," present-participle adjective from act (v.). Meaning "performing temporary duties" is from 1797.

c. 1600, "performance of deeds;" 1660s, "performance of plays;" verbal noun from present participle of act (v.). Acting out "abnormal behavior caused by unconscious influences" is from 1945 in psychiatry.

"sentence passed by the Inquisition" (plural autos-da-fé), 1723, from Portuguese auto-da-fé "judicial sentence, act of the faith," especially the public burning of a heretic, from Latin actus de fide. The elements are auto "a play," in law, "an order, decree, sentence," from Latin actus (see act (v.)), de "from, of" (see de), fides "faith" (from PIE root *bheidh- "to trust, confide, persuade"). The Spanish form is auto-de-fe, but the Portuguese form took hold in English, perhaps through popular accounts of the executions following the earthquake of 1755.

"to act together," c. 1600, from co- + act (v.). Related: Co-action; co-active; co-actor.

"act in opposition, hinder or defeat by contrary action," 1670s, from counter- + act (v.). Related: Counteracted; counteracting; counteractive; counteraction.

early 15c., "act the part of, represent in performance," from en- (1) "make, put in" + act (v.). Meaning "decree, establish, sanction into law" is from mid-15c. Related: Enacted; enacting.

"act on each other, act reciprocally," 1805, from inter- + act (v.). Related: Interacted; interacting.

of a play, "consisting of a single act," 1888, from one + act (n.).

1610s, "to go too far in action," from over- + act (v.). Meaning "play a part with too much emphasis, act (a part) with an extravagant and unnatural manner, chew the scenery" is from 1630s. Related: Overacted; overacting.

1640s, "to exert, as a thing acted upon, an opposite action upon the agent," from re- "back" + act (v.). Related: Reacted; reacting (1610s). For sense development, see reaction. The verb meaning "to perform again, do a second time" (often written re-act and given full pronunciation of the prefix to distinguish it from react) is from 1650s, from the "again" sense in re-.