The Grove Dictionary of music and musicians is more pedantic:
A composite form of vocal music consisting of a group of individually complete songs for solo or ensemble voices, with or without instrumental accompaniment.may relate a series of events, or a series of impressions, or may simply be a goup of songs unified by mood.
The texts may be by a single author or from different sources.

Liederzyklus, sangsyklus, etc., depending on language.

Some song cycles are songs without words, just the related tunes.