The closing price is the official final price of a security for a trading session, as determined by the exchange on which it is listed. It is not always the price of the last trade. Many exchanges set it through a closing auction, in which buy and sell orders are collected and matched at a single price, while others use a volume-weighted average of trades in the final minutes.
The closing price is the most widely used reference in markets. Funds use it to value their holdings, index providers use it to compute index levels, and clearing houses use it for mark-to-market and margin calculations. It is one of the four prices in a daily bar, alongside the opening price, high price and low price. Historical data often gives an adjusted close, restated for dividends and events such as a stock split, so that past returns are measured correctly.