The Collaborative International Dictionary of English v.0.48:

pachyderma \pachyderma\ (p[a^]k"[i^]*d[~e]r"m[.a]), n. (Med.)
   An abnormal thickening of the skin (usually unilateral on an
   extremity) caused by congenital enlargement of lymph vessel
   and lymph vessel obstruction.

   Syn: pachydermatosis, pachydermia, nevoid elephantiasis.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

pachyderma
    n 1: thickening of the skin (usually unilateral on an extremity)
         caused by congenital enlargement of lymph vessel and lymph
         vessel obstruction [syn: nevoid elephantiasis,
         pachyderma]