An exercise window is the period of time defined by a company that an employee may sell their vested stock options upon leaving that company. For example, if your company has a ten-year exercise window, then you will be able to sell your stock options for ten years after you have left. If you have not sold those stock options within ten years, then you forfeit your ownership of those vested stock options.
Exercise selection on Investopedia
Extending Stock Option Exercise Window Guide at Triplebyte
Zach Holman's personal blog on exercise windows
Zach Holman's open repository on extended exercise windows
Scott Kupor's Recommendations for Startup Employee Option Plans at Andreessen Horowitz
Scott Kupor's The Lack of Options for (Startup Employees’) Options at Andreessen Horowitz