Other forms: alacrities
Someone with alacrity shows cheerful willingness and eager behavior, like a kid whose mother has told him he can buy anything in a candy store.
While the noun alacrity normally refers to someone's peppy behavior, it can also describe a certain mood or tempo of a musical composition, indicating how the music should be played. Alacrity comes from the Latin alacritas, and the Italian musical term allegro is a near relation.