A mutual fund that sells shares of ownership and uses the proceeds to purchase short-term, high-quality debt securities such as Treasury bills, negotiable certificates of deposit, and commercial paper. Income earned by shareholders is received in the form of additional shares of stock in the fund (usually priced at $1 each). Although no fees are generally charged to purchase or redeem shares in a money market fund, an annual management charge is levied by the fund's advisers. This investment pays a return that varies with short-term interest rates. Both taxable and tax-exempt varieties of money market funds are offered. See also average maturity.