In music, a song is a composition for voice or voices, performed by singing. A song may be accompanied by musical instruments, or it may be unaccompanied, as in the case of a cappella songs. The lyrics (words) of songs are typically of a poetic, rhyming nature, though they may be religious verses or free prose.
A song may be for a solo singer, a duet, trio, or larger ensemble involving more voices. Songs with more than one voice to a part are considered choral works. 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 are by purpose (sacred vs secular), by style (dance, ballad, Lied, etc.), or by time of origin (Renaissance, Contemporary, 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".