What is an Investment bank?

An investment bank (IB) is a financial intermediary that performs a variety of services. Investment banks specialize in large and complex financial transactions, such as underwriting, acting as an intermediary between a securities issuer and the investing public, facilitating mergers and other corporate reorganizations, and acting as a broker and/or financial advisor for institutional clients.
www.investopedia.com/terms/i/investmentbank.asp