A position that is taken in the market and still active (not closed). This involves an investment (e.g., a share of stock, a bond, etc.) that is a subject matter of a market deal/ trade for which there is still no opposite position entered into by a trader/ investor

For example, an investor who has purchased, or is long, 500 shares of XYX is said to have an open position in that share. The position will still be open until the investor places an order to sell these shares and gets the the order filled by a market participant.

The opposite of an open position is a closed position.