1. One who is devoted to a particular occupation or branch of study or research: an antiquarian who is a specialist in 17th-century Dutch ceramics.

2. A physician whose practice is limited to a particular branch of medicine or surgery, especially one who is certified by a board of physicians: a specialist in oncology.

3. Any of several noncommissioned ranks in the US Army that correspond to that of corporal through sergeant first class.

4. A species with narrow habitat or food requirements: specialists that feed only on carrion.