The Collaborative International Dictionary of English v.0.48:

Cassock \Cas"sock\, n. [F. casaque, fr. It. casacca, perh. fr.
   L. casa cottage, in It., house; or of Slavic origin.]
   [1913 Webster]
   1. A long outer garment formerly worn by men and women, as
      well as by soldiers as part of their uniform.
      [1913 Webster]

   2. (Eccl.) A garment resembling a long frock coat worn by the
      clergy of certain churches when officiating, and by others
      as the usually outer garment.
      [1913 Webster]

WordNet (r) 3.0 (2006):

cassock
    n 1: a black garment reaching down to the ankles; worn by
         priests or choristers

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "cassock":
   churchman, cleric, clerical, clerk, confessor, cure, divine,
   ecclesiastic, father, father confessor, father in Christ, gallach,
   minister, padre, parish priest, parson, penitentiary, preacher,
   presbyter, priest, spiritual director, spiritual father