broker-dealer. A broker is a person or company engaged in the business of effecting transactions in securities for the account of others, often paid by commission. A dealer is any person or company engaged in the business of buying and selling securities for their own account. A broker-dealer is a firm that acts as both a broker and a dealer. bull market. A period during which a majority of securities prices in a particular market (such as the stock market) rise substantially. Contrast bear market.