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An investment bank is a financial institution that assists individuals, corporations, and governments in raising financial capital by underwriting or acting as the client's agent in the issuance of securities (or both). (Wikipedia)

After the Great Depression, through the Glass–Steagall Act, the U.S. Congress required that commercial banks only engage in banking activities, whereas investment banks were limited to capital market activities (Wikipedia)

A financial intermediary is an institution or individual that serves as a middleman among diverse parties in order to facilitate financial transactions. Common types include commercial banks, investment banks, stockbrokers, pooled investment funds, and stock exchanges. (Wikipedia)