<p><strong>Noun</strong></p><ul><li>a person devoted to one subject or to one particular branch of a subject or pursuit.</li><li>a medical practitioner who devotes attention to a particular class of diseases, patients, etc.</li><li>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.</li><li>Stock Exchange: a member of an exchange who buys and sells a single stock or a particular group of stocks in their own name or for other stockbrokers and thus helps maintain the market in those securities on that exchange.</li></ul>