The Collaborative International Dictionary of English v.0.48:

Iambus \I*am"bus\, n.; pl. L. Iambi, E. Iambuses. [L.
   iambus, Gr. ?; prob. akin to ? to throw, assail (the iambus
   being first used in satiric poetry), and to L. jacere to
   throw. Cf. Jet a shooting forth.] (Pros.)
   A foot consisting of a short syllable followed by a long one,
   as in [a^]m[=a]ns, or of an unaccented syllable followed by
   an accented one, as invent; an iambic. See the Couplet under
   Iambic, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

iambus
    n 1: a metrical unit with unstressed-stressed syllables [syn:
         iamb, iambus]