The Collaborative International Dictionary of English v.0.48:

Specialization \Spe`cial*i*za"tion\, n.
   1. The act of specializing, or the state of being
      spezialized.
      [1913 Webster]

   2. (Biol.) The setting apart of a particular organ for the
      performance of a particular function. --Darwin.
      [1913 Webster]

WordNet (r) 3.0 (2006):

specialization
    n 1: the act of specializing; making something suitable for a
         special purpose [syn: specialization, specialisation]
    2: the special line of work you have adopted as your career;
       "his specialization is gastroenterology" [syn:
       specialization, specialisation, specialty,
       speciality, specialism]
    3: (biology) the structural adaptation of some body part for a
       particular function; "cell differentiation in the developing
       embryo" [syn: specialization, specialisation,
       differentiation]

Moby Thesaurus II by Grady Ward, 1.0:

75 Moby Thesaurus words for "specialization":
   alteration, analysis, anatomization, area, art, atomization, bag,
   business, calling, career, career building, careerism, change,
   craft, cup of tea, demarcation, desynonymization, differencing,
   differentiation, discrimination, disequalization, disjunction,
   distinction, distinguishment, diversification, division, field,
   forte, game, handicraft, individualization, individuation,
   itemization, lifework, line, line of business, line of work,
   localization, long suit, main interest, manner, metier, mission,
   modification, mystery, number, occupation, particularization,
   peculiarization, personalization, pet subject, practice,
   profession, pursuit, racket, segregation, separation,
   severalization, severance, specialism, speciality, specialty,
   strong point, style, technicality, thing, trade, type, variation,
   vocation, walk, walk of life, way, weakness, work