What term describes a regularly repeated line or group of lines in a poem or song?

The term that describes a regularly repeated line or group of lines in a poem or song is "refrain." A refrain often appears at the end of a stanza or between stanzas, serving both a thematic and rhythmic purpose. It enhances the musical quality of the poem or song and reinforces key ideas or emotions throughout the piece. This repetition creates a familiar structure for the audience and can underscore the main message being conveyed.

In contrast, a stanza refers to a grouped set of lines in a poem, similar to a paragraph in prose, which may not necessarily involve repetition. A couplet specifically refers to a pair of lines, usually with end rhymes, and is itself a building block of stanzas but does not imply repetition. Verse is a broader term that can refer to any single line of poetry, the stanzaic form, or poetry in general, and doesn't specifically indicate the element of repetition. Thus, "refrain" is the most accurate term in this context.