A quatrain is a group of four lines in a poem. Those four lines belong together as one block. They often rhyme — patterns like AABB or ABAB are common — though they do not have to rhyme. A lot of English poems, hymns, and song verses are built from quatrains stacked one after another. Teachers use the word when they talk about how a poem is put together. It is a school-and-poetry word, not a street word. If you only need to say “four lines,” you can say that; quatrain is the precise name for that four-line unit.

四行詩