What is Index Fund?

Legal Definition
An index fund (also index tracker) is a mutual fund or exchange-traded fund (ETF) designed to follow certain preset rules so that the fund can track specified basket of underlying investments. Those rules may include tracking prominent indexes like the S&P 500 or the Dow Jones Industrial Average or implementation rules, such as tax-management, tracking error minimization, large block trading or patient/flexible trading strategies that allows for greater tracking error, but lower market impact costs. Index funds may also have rules that screen for social and sustainable criteria.

An index fund’s rules of construction clearly identify the type of companies suitable for the fund. The most commonly known index fund, the S&P 500 Index Fund, is based on the rules established by S&P Dow Jones Indices for their S&P 500 Index. Equity index funds would include groups of stocks with similar characteristics such as the size, value, profitability and/or the geographic location of the companies. A group of stocks may include companies from the United States, Non-US Developed, emerging markets or Frontier Market countries. Additional index funds within these geographic markets may include indexes of companies that include rules based on company characteristics or factors, such as companies that are small, mid-sized, large, small value, large value, small growth, large growth, the level of gross profitability or investment capital, real estate, or indexes based on commodities and fixed-income. Companies are purchased and held within the index fund when they meet the specific index rules or parameters and are sold when they move outside of those rules or parameters. Think of an index fund as an investment utilizing rules-based investing. Some index providers announce changes of the companies in their index before the change date and other index providers do not make such announcements.

One index provider, Dow Jones Indexes, has 130,000 indices. Dow Jones Indexes says that all its products are maintained according to clear, unbiased, and systematic methodologies that are fully integrated within index families.

As of 2014, index funds made up 20.2% of equity mutual fund assets in the US. Index domestic equity mutual funds and index-based exchange-traded funds (ETFs), have benefited from a trend toward more index-oriented investment products. From 2007 through 2014, index domestic equity mutual funds and ETFs received $1 trillion in net new cash, including reinvested dividends. Index-based domestic equity ETFs have grown particularly quickly, attracting almost twice the flows of index domestic equity mutual funds since 2007. In contrast, actively managed domestic equity mutual funds experienced a net outflow of $659 billion, including reinvested dividends, from 2007 to 2014.
-- Wikipedia
Legal Definition
A mutual fund that uses market index to guarantee returns. This is also called indexing or passive investment strategy.