specialist

One who is devoted to a particular occupation or branch of study or research.

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

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

A species with narrow habitat or food requirements.

A person who devotes himself to a particular branch of a profession, science, or art; one who has a special knowledge of some particular subject: thus, ophthalmologists, neurologists, or gynecologists are specialists in medicine.