A mutual fund is a professionally-managed firm of collective investments that pools money from many investors and invests it in stocks, bonds, short-term money market instruments, and/or other securities. In a mutual fund, the fund manager, who is also known as the portfolio manager, trades the fund's underlying securities, realizing capital gains or losses, and collects the dividend or interest income. The investment proceeds are then passed along to the individual investors. The value of a share of the mutual fund, known as the net asset value per share (NAV), is calculated daily based on the total value of the fund divided by the number of shares currently issued and outstanding.

Legally known as an "open-end company" under the Investment Company Act of 1940 (the primary regulatory statute governing investment companies), a mutual fund is one of three basic types of investment companies available in the United States. Outside of the United States (with the exception of Canada, which follows the U.S. model), mutual fund may be used as a generic term for various types of collective investment vehicle. In the United Kingdom and western Europe (including offshore jurisdictions), other forms of collective investment vehicle are prevalent, including unit trusts, open-ended investment companies (OEICs), SICAVs and unitized insurance funds. In Australia and New Zealand the term "mutual fund" is generally not used; the name "managed fund" is used instead.