A song cycle is

Answer 1

Basically it is a group of songs to be performed in sequence as a single entity - usually by the same composer and set to the words from the same poet.

(apex) a group of songs connected to each other by a common idea, emotion, or story.

Answer 2

a group of songs connected to each other by a common idea, emotion, or story.

Answer 3

Usually this refers to a series of Opera. The most famous example of such is Wagner's Ring cycle.