What is Improvise?

Improvise definition and meaning on Dictionary terms:
verb (used with object), imA.proA.vised,
to compose and perform or deliver without previous preparation; extemporize: to improvise an acceptance speech.
to compose, play, recite, or sing (verse, music, etc.) on the spur of the moment.
to make, provide, or arrange from whatever materials are readily available: We improvised a dinner from yesterday’s leftovers.

verb (used without object), imA.proA.vised,
to compose, utter, execute, or arrange anything extemporaneously: When the actor forgot his lines he had to improvise.