1. An unexecuted order that is still valid.
2. The start of trading on a securities exchange.
Taobiz explains Open
1. Orders that are entered by investors but not yet transacted are deemed to be open until they expire or are filled.
2. Different exchanges will have different opening times. For instance, the New York Stock Exchange and the Nasdaq open at 9.30am EST, whereas the American Stock Exchange opens at 9am EST for options on debt securities.