cadenza
A cadenza is an extended, improvisatory style section added near the end of a movement and often based on the themes of the piece in which it appears. The original purpose of a cadenza (Italian for "cadence") was to give singers or players a chance to show off their technical skill and ability to improvise, but Mozart began the practice of providing written cadenzas for some of his piano concertos.
In a concerto the end of the cadenza is marked by a dominant 7th chord.