The Collaborative International Dictionary of English v.0.48:

Mink \Mink\ (m[i^][ng]k), n. [Cf. 2d Minx.]
   1. (Zool.) A carnivorous mammal of the genus Mustela
      (foremrly Putorius), allied to the weasel. The European
      mink is Mustela lutreola. The common American mink
      (Mustela vison) varies from yellowish brown to black.
      Its fur is highly valued. Called also minx, nurik, and
      vison.
      [1913 Webster +PJC]

   2. The fur of the mink[1]. Together with sable, it is one of
      the most expensive furs not taken from endangerd species.
      When the fur is taken from animals grown on a farm, it
      called ranch mink.
      [PJC]

WordNet (r) 3.0 (2006):

Mustela vison
    n 1: usually rich dark brown [syn: American mink, Mustela
         vison]