The Collaborative International Dictionary of English v.0.48:
Quatrain \Quat"rain\, n. [F., fr. quatre four, L. quattuor, quatuor. See Four.] (Pros.) A stanza of four lines rhyming alternately. --Dryden. [1913 Webster]WordNet (r) 3.0 (2006):
quatrain n 1: a stanza of four lines