Definition of 'Open'
1. An unexecuted order that is still valid. An open order is one that has been placed either electronically or with a broker and has not been filled due to illiquidity or the terms of the position not being fulfilled
2. The start of trading on a securities exchange. The open on a trading exchange signals the start of an official day for the exchange, and that buy and sell transactions can commence for the business day.
Investopedia explains 'Open'
1. Orders that are entered by investors but not transacted are deemed to be open until they expire or are filled. Different orders, such as limit or stop order can influence if or when an order is filled.
2. Different exchanges will have different opening times. For instance, the New York Stock Exchange (NYSE) and the Nasdaq open at 9.30 am EST, whereas the American Stock Exchange (AMEX) opens at 9 am EST for options on debt securities.