Equity indices, or stock indices as they are also commonly known, are actual stock market indexes, which measure the value of a specific section of a stock market. They are calculated based on a weighted average of the prices of selected stocks

which belong to the actual category that they represent. Stock indices can represent a specific stock market such as NASDAQ, or they can represent a specific set of the largest companies of a nation such as the American S&P 500, the British FTSE 100, or the Japanese Nikkei 225.

The purpose of the indices is to show the general direction of a specific stock market or of the general economy of a nation. However, since stock indices are composed of a basket of companies they can be very much affected by a big move of a specific company or by a big move of a specific sector of trade.

The actual weight given to a stock index from the underlying basket of stocks varies amongst the various indices, which means that not all use the same criteria to derive the end result. The two main ways to calculate the actual weight a specific underlying stock produces to the index itself is price weighting and capitalization weighting.