Structure
Quatrain
From French quatre: four. The quatrain is the building block of the sonnet, the ballad, and the hymn. Omar Khayyam's Rubáiyát popularised it in Fitzgerald's translation (1859).
Definition
A stanza or complete poem of four lines — the most common stanza unit in English verse, with many rhyme schemes.
Example
ABAB (heroic stanza), ABBA (envelope), AABB (couplet pair), ABCB (ballad stanza).