Impromptu (adjective)

1. Done or spoken without prior preparation; improvised.
2. Made or done on the spur of the moment; spontaneous.
3. Not planned or prepared in advance; extemporaneous.

Origin:

From the french word "impromptu" meaning "done without prior preparation".

Examples:

1. He gave an impromptu speech at the event.
2. She played an impromptu song on the piano.
3. The impromptu meeting was called to discuss the urgent matter.
4. The impromptu performance was a surprise to everyone.
5. The impromptu decision was made based on the circumstances.