The Collaborative International Dictionary of English v.0.48:

Soutane \Sou`tane"\, n. [F., fr. Sp. sotana, or It. sottana, LL.
   subtana, fr. L. subtus below, beneath, fr. sub under.] (Eccl.
   Costume)
   A close garnment with straight sleeves, and skirts reaching
   to the ankles, and buttoned in front from top to bottom;
   especially, the black garment of this shape worn by the
   clergy in France and Italy as their daily dress; a cassock.
   [1913 Webster]

WordNet (r) 3.0 (2006):

soutane
    n 1: a long cassock with buttons down the front; worn by Roman
         Catholic priests