(1):

(n.) Music sung or performed in the open air at nights; - usually applied to musical entertainments given in the open air at night, especially by gentlemen, in a spirit of gallantry, under the windows of ladies.

(2):

(v. i.) To perform a serenade.

(3):

(v. t.) To entertain with a serenade.

(4):

(n.) A piece of music suitable to be performed at such times.