im ‧ promp ‧ tu /ɪmˈprɒmptjuː $ ɪmˈprɑːmptuː/ BrE AmE adjective
[ Date: 1600-1700 ; Language: French ; Origin: Latin in promptu 'in readiness' ]
done or said without any preparation or planning
impromptu speech/party/meeting etc
The band gave an impromptu concert.
—impromptu adverb