A stock broker or stockbroker is someone or a firm who performs transactions in financial instruments on a stock market either (1) as an agent of clients who are unable or unwilling to trade for themselves or (2) as a principal i.e. on his/her/its own behalf.

When acting as an agent, the stockbroker charges a flat fee and/or a percentage-based commission for undertaking the trade.

A stock broker is also known as a stock trader, or simply as a trader.