1
adj ad-lib, extemporaneous, extempore, extemporized, improvised, offhand, off the cuff (informal) spontaneous, unpremeditated, unprepared, unrehearsed, unscripted, unstudied
2
adv ad lib, off the cuff (informal) off the top of one's head (informal) on the spur of the moment, spontaneously, without preparation
Antonyms
, adj considered, planned, premeditated, prepared, rehearsed