The Collaborative International Dictionary of English v.0.48:

Specialist \Spe"cial*ist\, n.
   One who devotes himself to some specialty; as, a medical
   specialist, one who devotes himself to diseases of particular
   parts of the body, as the eye, the ear, the nerves, etc.
   [1913 Webster]

WordNet (r) 3.0 (2006):

specialist
    n 1: an expert who is devoted to one occupation or branch of
         learning [syn: specialist, specializer, specialiser]
         [ant: Renaissance man, generalist]
    2: practices one branch of medicine [syn: specialist, medical
       specialist]