A stock index or stock market index is a measurement of the value of a section of the stock market. It is computed from the prices of selected stocks (typically a weighted average). It is a tool used by investors and financial managers to describe the market, and to compare the return on specific investments.
An index is a mathematical construct, so it may not be invested in directly. But many mutual funds and exchange-traded funds attempt to "track" an index (see index fund), and those funds that do not may be judged against those that do.