A firm, acting as an underwriter or an agent, who serves as intermediary between an issuer of new securities and the investing public. The usual practice is for one or more investment bankers to form a syndicate to buy a corporation's new issue and then sell the issue to individuals and institutions--commonly called a "firm commitment underwriting". In a provisional arrangement--called "best effort"--the investment banker acts as an agent rather than principal and markets a new issue without underwriting it. Under another provisional arrangement--called "standby commitment"--the investment banker agrees to buy for resale any securities not taken by existing holders of rights.
If a client relationship exists, the investment banker's role starts with pre-underwriting counseling and continues after the distribution of securities is completed by offering ongoing advice and guidance. Some underwriting responsibilities include preparing the SEC registration statement, pricing the securities, forming and managing the syndicate, and pegging (stabilizing) the price of the issue during the offering and distribution period.
Besides new securities offerings, investment bankers manage the distribution of secondary offerings, maintain markets for already distributed securities and act as finders for private placements. Most investment bankers also maintain broker-dealer operations that serve wholesale and retail clients in brokerage and advisory capacities.

This definition is in context to Frauds and Scams. See more contextual defintions for Investment Banker.