In common usage, a song is a short piece of verse set to music. A song can be as simple as a nursery rhyme, or as complex as some progressive rock explorations that raise the question of where the dividing line is between a song and a classical work.
Pieces of instrumental music, without words, may also be referred to as songs, particularly when they are enjoyed in a similar context to songs with words - for example, some 'songs' by the ska group Madness are mostly or completely instrumental.
In the Bible, passages written in verse are sometimes called songs. The most obvious of these is the Song of Solomon, but they also include the Song of Deborah in the Book of Judges, and indeed the Psalms. Some or all of these writings may have been sung by contemporary readers, but the original tunes, if they ever existed, are unknown today.
This older usage of 'song' can also be applied to secular poetry; Italian poets such as Dante and Petrarch frequently refer in their works to 'my song' (following the tradition of classical poets like Homer), even though their works were always written to be read rather than sung.
In the liturgy used by some Christian denominations, certain passages from the Gospel are referred to as songs. The most frequently encountered are the Song of Mary (also called the Magnificat) and the Song of Simeon (also called the Nunc Dimitis).