The Collaborative International Dictionary of English v.0.48:

Ottava rima \Ot*ta"va ri"ma\ [It. See Octave, and Rhyme.]
   (Pros.)
   A stanza of eight lines of heroic verse, with three rhymes,
   the first six lines rhyming alternately and the last two
   forming a couplet. It was used by Byron in "Don Juan," by
   Keats in "Isabella," by Shelley in "The Witch of Atlas," etc.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

ottava rima
    n 1: a stanza of eight lines of heroic verse with the rhyme
         scheme abababcc