specialisation

(noun) (biology) the structural adaptation of some body part for a particular function. Synonyms: specialization, differentiation. Example: "Cell differentiation in the developing embryo."

(noun) the act of specializing; making something suitable for a special purpose. Synonyms: specialization.

(noun) the special line of work you have adopted as your career. Synonyms: specialization, specialty, speciality, specialism. Example: "His specialization is gastroenterology."