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

A reduction in generality, usually for the sake of increased efficiency. If a piece of code is specialised for certain values of certain variables (usually function arguments), this is known as "partial evaluation". In a language with overloading (e.g. Haskell), an overloaded function might be specialised to a non-overloaded instance at compile-time if the types of its arguments are known.