The Collaborative International Dictionary of English v.0.48:
Iamb \I"amb\, n. [Cf. F. iambe. See Lambus.] An iambus or iambic. [R.] [1913 Webster]WordNet (r) 3.0 (2006):
iamb n 1: a metrical unit with unstressed-stressed syllables [syn: iamb, iambus]