impromptu

adj

1. (not-comparable)Improvised; without prior preparation, planning or rehearsal.

   “The party began with an impromptu rendition of 'Happy Birthday'.”

   “an impromptu speech”

   “I can appreciate that Macca’s band plays faithful to the source material, never putting too obvious a personal spin on the music. It’s sometimes frustrating, especially when the solo part in “Get Back” is screaming for some impromptu finesse, but it’s mostly fair.”

adv

1. (not-comparable)Extemporaneously; without prior preparation, planning, or rehearsal.

   “happened impromptu”

   “was done impromptu”

noun

1. A short musical composition for an informal occasion often with the character of improvisation and usually to be played solo.

   “The second impromptu is a dance-like Allegretto in A flat major, with a trio in D flat major employing arpeggiated textures.”

2. (broadly)Any composition, musical or otherwise, that is created on the spot without preparation.