From The Art and Popular Culture Encyclopedia
A refrain (from Vulgar Latin refringere, "to repeat", and later from Old French refraindre) is the line or lines that are repeated in music or in verse; the "chorus" of a song. Poetic fixed forms that feature refrains include the villanelle, the virelay, and the sestina.
The use of refrains is particularly associated with where the verse-chorus-verse song structure typically places a refrain in almost every song. The refrain or chorus often sharply contrasts the verse melodically, rhythmically, and harmonically, and assumes a higher level of dynamics and activity, often with added instrumentation. Chorus form, or strophic form, is a sectional and/or additive way of structuring a piece of music based on the repetition of one formal section or block played repeatedly.