A schedule that sets forth the timeframe and/or performance goals that must be met before stock options or stock appreciation rights become exercisable, or before restricted stock, restricted stock units, or performance shares cease being subject to forfeiture and come under your control.

The schedule is usually based on the length of time you work at your company and occasionally can be accelerated for meeting performance targets (e.g., company stock price hits X price and stays at that level for 60 days). In some option plans at pre-IPO companies you can exercise unvested options, subject to a company repurchase right under a vesting-like schedule.