Impromptu means done on the spot without preparation. An impromptu speech, an impromptu party, entirely impromptu.
Many things called impromptu were quietly prepared, which is part of the word's use. In music an impromptu is a short piece written to sound as if it were improvised, which is the same joke set down.
1adjectiveimpromptuDone on the spot without preparation.
“When the power went out, the guests held an impromptu sing-along by candlelight.”
impromptu is a quality.
From impromptu.
Etymology from Wiktionary, CC BY-SA