A person on the floor of an exchange who trades securities on behalf of a brokerage or investor. He/she may only make trades as directed by the client, or may have some leeway in the trades he/she makes. For his/her services, this person is paid a commission, which is a percentage of value of each transaction the broker makes. In order to guard against abuse, commission brokers working with discretionary accounts are required to follow suitability rules, which mandate that the broker only make trades in good faith, that is, only makes trades a "reasonable person" would make.
An employee of a member firm of an organized securities exchange who transacts orders for the firm or its customers on the exchange floor. Orders flow to the commission broker from the firm's trading desk or from its registered representatives.