A historical price is a recorded past price of a security, currency or contract, as opposed to the live price at which it can be traded now. Price history is stored at different levels of detail, from every trade and quote to one-minute bars and daily records of the open, high, low and closing price. It is the raw material for charts, for backtesting a trading rule and for measures such as realized volatility.
Raw history is usually adjusted before use. A stock split or a large dividend causes a jump in the quoted price that is not a gain or loss to holders, so vendors publish adjusted series that remove it. Futures histories must be stitched together across contract months. Poor history misleads: missing delisted companies creates survivorship bias, and prices that were corrected later can make a test look better than real trading would have been.