A compositional form; a composer takes already existing text (or writes some himself) and then sets the text to music in a lyrical song-like manner.

Song cycles may be in any language and through the years have begun to show up in many different sizes of orchestration - from piano and voice to full orchestra and voice.

Traditionally, a song cycle's contents are all related in some way, either all being part of the same story, or from the same author, etc. Only one voice sings during each song, although voices may alternate or change between songs. If more than one voice sings during a song it slides into opera.

With the advent of the musical academia from the dawn of the 20th century until today, I imagine that pretty much anything that features an instrument, divisions, and at least one voice may be called a song cycle. A true song cycle adheres to a symphonic form which dictates ranges, voice leading, orchestration, subject matter, and structure.