Some of the best kinds of parties are impromptu ones, when you decide at the last minute to get together. The adjective impromptu describes things done or said without previous thought or preparation.
Impromptu is also used as an adverb: Most people are not able to speak impromptu in front of an audience. This word was borrowed from French and ultimately derives from the Latin in prōmptū ("at hand, in readiness") from in ("in") plus a form of prōmptus ("readiness"). As you can guess from the spelling, the adjective and verb prompt are related to impromptu; they are from the same Latin verb, prōmere, "to bring forth."