The Collaborative International Dictionary of English v.0.48:

Cataplasm \Cat"a*plasm\, n. [L. cataplasma, Gr. ?, fr. ? to
   spread over; kata` down, wholly + ? to form, mold.] (Med.)
   A soft and moist substance applied externally to some part of
   the body; a poultice. --Dunglison.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cataplasm
    n 1: a medical dressing consisting of a soft heated mass of meal
         or clay that is spread on a cloth and applied to the skin
         to treat inflamed areas or improve circulation etc. [syn:
         poultice, cataplasm, plaster]