The price at which a security first trades upon the opening of an exchange on a given trading day.

Quite commonly, a security's opening price will not be identical to its closing price. This is due to after-hours trading and to changes in investor valuations or expectations of the security occurring outside of trading hours. The opening price of a security serves as a benchmark for the day: if the security finishes trading at a higher price it is said to have closed up, and if it finishes trading below the opening price it is said to have closed down.