The Collaborative International Dictionary of English v.0.48:

dermatologist \der`ma*tol"o*gist\, n.
   One who discourses on the skin and its diseases; one versed
   in dermatology; especially, a physician with specialized
   training in dermatology, licensed to practise as a specialist
   in treating diseases of the skin.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

dermatologist
    n 1: a doctor who specializes in the physiology and pathology of
         the skin [syn: dermatologist, skin doctor]