A market intermediary who brings together buyer and seller for a commission paid by the initiator of the transaction or by both sides.
The broker does not take market positions itself.
More specifically, an individual or a firm (also called broking house) that acts as an agent for investors by dealing in securities.
Usually, the broker will charge commissions (called brokerage) for providing advisory and trading services. Brokers do not buy or sell on their own account but act as agents for clients.