The Collaborative International Dictionary of English v.0.48:

Specialize \Spe"cial*ize\, v. t.
   1. To mention specially; to particularize.
      [1913 Webster]

   2. To apply to some specialty or limited object; to assign to
      a specific use; as, specialized knowledge.
      [1913 Webster]

   3. (Biol.) To supply with an organ or organs having a special
      function or functions.
      [1913 Webster]

WordNet (r) 3.0 (2006):

specialize
    v 1: become more focus on an area of activity or field of study;
         "She specializes in Near Eastern history" [syn:
         specialize, specialise, narrow, narrow down] [ant:
         branch out, broaden, diversify]
    2: be specific about; "Could you please specify your criticism
       of my paper?" [syn: specify, particularize,
       particularise, specialize, specialise] [ant:
       generalise, generalize]
    3: suit to a special purpose; "specialize one's research"; "this
       kind of beak has become specialized in certain Galapagos
       finches" [syn: specialize, specialise]
    4: devote oneself to a special area of work; "She specializes in
       honey bees"; "This baker specializes in French bread" [syn:
       specialize, specialise]
    5: evolve so as to lead to a new species or develop in a way
       most suited to the environment [syn: speciate,
       differentiate, specialize, specialise]

Moby Thesaurus II by Grady Ward, 1.0:

86 Moby Thesaurus words for "specialize":
   analyze, anatomize, assign, atomize, be into, be strong in, bound,
   change, chop logic, condition, confine, contain, copyright,
   denominate, descend to particulars, designate, desynonymize,
   detail, determine, difference, differentiate, discipline,
   discriminate, disequalize, disjoin, distinguish, diversify, divide,
   draw the line, enumerate, feature, fix, follow, go in for,
   hedge about, indicate, individualize, individuate, inventory,
   itemize, limit, list, major in, make a distinction, mark, mark off,
   mark out, mention, minor in, moderate, modify, name, narrow,
   particularize, patent, peculiarize, personalize, pick out,
   pin down, point out, precise, pursue, qualify,
   refine a distinction, register, restrain, restrict, scant,
   segregate, select, separate, set, set apart, set off, sever,
   severalize, signify, specialize in, specify, spell out,
   split hairs, state, stint, stipulate, straiten, vary