Refrain is a repeating verse, a closing stanza or a group of them. There can be several. Different dictionaries give several other interpretations of the term, and we will discuss them below.
Rondo
According to the explanatory dictionary of Efremova, the refrain is part of a certain vocal work, which has a couplet form. This fragment is repeated in a constant form and acts as a chorus. Also this source indicates that the play with the refrain is called "rondo". In this case, this fragment repeats itself many times, alternating with various episodes.
Poem
Now let us consider the interpretations given in the "Small Academic Dictionary". In literature, a refrain is a verse or a group that repeats within the framework of a single work. In music, this concept is responsible for a fragment of the vocal unit of the couplet form, which is repeated in a constant form and is a chorus. Chernyshevsky's work "What to do" mentions a brisk, bold song with a cheerful melody with two or three sad notes that disappeared in the refrain and disappeared in the final verse.
Verse
Now let us look at the interpretation given in the Dictionary of Foreign Terms of the Russian Language. According to this source, the refrain is the repetition of identical words after each verse or stanza. Also it can be called a chorus (this is how the word refrain is translated from French). A similar definition is also given in Kuznetsov's Explanatory Dictionary. Speech in this case is about the verse or their group, which is repeated in a song or poem. In the first case, it is called a chorus. This term is also used if one wants to indicate a frequent repetition of something similar. As a rule, the refrain is included in the work at least three times. There is a similar definition in the explanatory dictionary of Ozhegov.