The Collaborative International Dictionary of English v.0.48:

Finback \Fin"back`\, n. (Zool.)
   Any whale of the genera Sibbaldius, Bal[ae]noptera, and
   allied genera, of the family Bal[ae]nopterid[ae],
   characterized by a prominent fin on the back. The common
   finbacks of the New England coast are Sibbaldius
   tectirostris and S. tuberosus.
   [1913 Webster]

WordNet (r) 3.0 (2006):

finback
    n 1: large flat-headed whalebone whale having deep furrows along
         the throat; of Atlantic and Pacific [syn: finback,
         finback whale, fin whale, common rorqual,
         Balaenoptera physalus]