a poem of fixed form, French in origin, consisting usually of five three-line stanzas and a final four-line stanza and having only two rhymes throughout
See villanelle in American Heritage Dictionary 4
(vĭlˌə-nĕlˈ)
noun
A 19-line poem of fixed form consisting of five tercets and a final quatrain on two rhymes, with the first and third lines of the first tercet repeated alternately as a refrain closing the succeeding stanzas and joined as the final couplet of the quatrain.