Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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