Refrain

A refrain is a phrase or a verse that reoccurs many times throughout a poem or a song, especially at the end of every stanzas. Note that sometimes the refrain may be the same as the name of the song. However, this is optional.

For example, in the happy birthday song, happy birthday is the refrain

Happy birthday to you
Happy birthday to you
Happy birthday dear (anonymous)
Happy birthday to you

source: Google Images