Definitions for "Improvise"
To compose, recite, or sing extemporaneously, especially in verse; to extemporize; also, to play upon an instrument, or to act, extemporaneously.
To bring about, arrange, do, or make, immediately or on short notice, without previous preparation and with no known precedent as a guide.
To invent, or provide, offhand, or on the spur of the moment; as, he improvised a hammer out of a stone.
manage in a makeshift way; do with whatever is at hand; "after the hurricane destroyed our house, we had to improvise for weeks"
To produce or make something from whatever is available.
( link / ) What the GM does when working without notes or pre-written adventure material, often as a response to something unexpected from the players.