Other forms: allegros
In music, allegro distinguishes a movement that's meant to be played very quickly. Your piano teacher might instruct you to try playing a piece allegro.
If you're reading sheet music and you see the word allegro, you'll know that particular section or movement should be played in a lively, spirited way. There are many Italian musical terms that describe or direct the tempo, or speed, of the music, and allegro is one of these. The word means "cheerful or gay" in Italian from the Latin root alacrem, "lively, cheerful, or brisk."