specialisation

noun4

1. •
   The act or process of specializing.
2. •
   The adaptation of an organism to a specific environment, or adaptation of an organ to a particular function.

   • Specialization is frequently divided into two categories largely based on methods of study: food specialization and specialization with respect to microhabitat. — 2012
3. •
   The area in which someone specializes.
4. •
   A proof, axiom, problem, or definition whose cases are completely covered by another, broader concept.

   • A triangle is a specialization of a polygon to one with exactly three sides.