a person who devotes himself or herself to one subject or to one particular branch of a subject or pursuit.

a medical practitioner who devotes attention to a particular class of diseases, patients, etc.

U.S. Army . an enlisted person of one of four grades having technical or administrative duties, the grades corresponding to those of corporal through sergeant first class but not requiring the exercise of command.

Stock Exchange . a member of an exchange who executes orders in his or her name or for other stockbrokers in a single stock or a particular group of stocks and thus helps maintain the market in those securities on that exchange.

noun

general practitioner

noun

ace , adept , authority , connoisseur , consultant , devotee , doctor , guru * , old hand * , old pro , pro * , professional , pundit , sage , savant , scholar , technician , veteran , virtuoso , expert , master , maven