song is a composition (musical composition) that contains vocal (human voice) parts ("lyrics") that are performed ("sung"), commonly accompanied by musical instruments, exception in the case of a cappella songs. The lyrics of songs are typically of a poetic (poetry), rhyming (rhyme) nature, although they may be religious verses (scripture) or free prose.

Songs are typically for a solo singer, though they may also be in the form of a duet, trio (trio (music)), or larger ensemble involving more voices. See part song. (Works with more than one voice to a part, however, are considered choral (choir).) Songs can be broadly divided into many different forms, depending on the criteria used. One division is between "art songs", "pop songs", and "folk songs". Other common methods of classification (Categorization) are by purpose (sacred vs secular), by style (music genre) (dance, ballad, Lied, etc.), or by time of origin (Renaissance, Contemporary (Contemporary music), etc.).

A song is a piece of music for accompanied or unaccompanied voice or voices or, "the act or art of singing," but the term is generally not used for large vocal forms including opera and oratorio. However, the term is, "often found in various figurative and transferred sense (e.g. for the lyrical second subject of a sonata...)." The word "song" has the same etymological root as the verb "to sing" and the OED defines the word to mean "that which is sung".