impromptu
adj. C1 Advanced US //ˌɪmˈpɹɑmptu// UK //ɪmpɹˈɒmptuː// im·promp·tu
adj. done or said without any preparation or planning. You use this when someone does something on the spot because they have to or want to.
adj. planned, organized, or performed without advance preparation; extemporaneous. Often used to describe speeches, performances, or social gatherings.
The team held an impromptu celebration after winning the game.
When the guest speaker failed to arrive, the professor gave an impromptu lecture on her recent research.
The jazz musicians engaged in an impromptu jam session that lasted until dawn, showcasing their ability to harmonize without a written score or prior rehearsal.
Borrowed from French impromptu.
Usage
Typically used attributively before a noun; can also follow linking verbs like 'be' or 'seem'.