to pursue some special line of study, work, etc.; have a specialty

The doctor specializes in gastroenterology.

Biology . (of an organism or one of its organs) to be adapted to a special function or environment.

to render special or specific; invest with a special character, function, etc.

to adapt to special conditions; restrict to specific limits.

to restrict payment of (a negotiable instrument) by endorsing over to a specific payee.

to specify; particularize.