specialization

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The act of specializing or the process of becoming specialized.
• noun Adaptation, as of an organ or a species, to a specific function or environment.
• noun A character or feature resulting from such adaptation.

from The Century Dictionary.

• noun The act or process of specializing; a making or fixing of special differences or requirements; differentiation.
• noun The state of being or becoming specialized; a condition of fixed or developed differentiation, as of parts, organs, or individuals, with reference to form, appearance, function, etc.
• noun In biology, that evolutionary process whereby parts or organs primitively indifferent or of common character become differentiated in form or function (usually in both); also, the result of such process or course of development; adaptive modification.
• noun Also spelled specialisation.

from the GNU version of the Collaborative International Dictionary of English.

• noun The act of specializing, or the state of being spezialized.
• noun (Biol.) The setting apart of a particular organ for the performance of a particular function.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The process of specializing.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (biology) the structural adaptation of some body part for a particular function
• noun the special line of work you have adopted as your career
• noun the act of specializing; making something suitable for a special purpose