A villanelle is a poetic form with nineteen lines and a repeating pattern and rhyme scheme. A villanelle is comprised of five tercets (three lines) followed by one four-line quatrain. The first and third lines of the opening tercet repeat in an alternating pattern creating the final line of each next tercet. In turn, those two lines then form the final two lines of the entire poem.
The rhyme scheme calls for repeating lines to rhyme, and the second line of every tercet must rhyme also. The pattern looks like this: A1 b A2 / a b A1 / a b A2 / a b A1 / a b A2 / a b A1 A2.
A well-known example of a villanelle is “Do Not Go Gentle Into that Good Night” by Dylan Thomas.