The definition of a mutual fund is a form of collective investment that pools money from many investors and invests their money in stocks, bonds, short-term money market instruments, and/or other securities. In a mutual fund, the fund 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 is 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 the term "mutual fund" is generally not used; the name "managed fund" is used instead. However, "managed fund" is somewhat generic as the definition of a managed fund in Australia is any vehicle in which investors' money is managed by a third party (NB: usually an investment professional or organization). Most managed funds are open-ended (i.e., there is no established maximum number of shares that can be issued); however, this need not be the case. Additionally the Australian government introduced a compulsory superannuation/pension scheme which, although strictly speaking a managed fund, is rarely identified by this term and is instead called a "superannuation fund" because of its special tax concessions and restrictions on when money invested in it can be accessed.