The Collaborative International Dictionary of English v.0.48:

Right whale \Right" whale`\ (Zool.)
   (a) The bowhead, Arctic, or Greenland whale (Balaena
       mysticetus), from whose mouth the best whalebone is
       obtained.
   (b) Any other whale that produces valuable whalebone, as the
       Atlantic, or Biscay, right whale (Balaena cisarctica),
       and the Pacific right whale (Balaena Sieboldii); a bone
       whale.
       [1913 Webster]

   Pygmy right whale (Zool.), a small New Zealand whale
      (Neobalaena marginata) which is only about sixteen feet
      long. It produces short, but very elastic and tough,
      whalebone.
      [1913 Webster]

WordNet (r) 3.0 (2006):

right whale
    n 1: large Arctic whalebone whale; allegedly the `right' whale
         to hunt because of its valuable whalebone and oil