The fixed per-share price an employee pays to exercise a stock option, set at the company's fair market value on the day the option is granted.
Engineers who joined Tesla around 2008 received options at strike prices below five dollars per share, set against the company's 409A at the time. By 2020 the split-adjusted share price had crossed several hundred dollars, turning four-year grants into life-changing exits for anyone who had hung on through the production hell years.