Vesting schedule
The timetable that specifies when options vest, or become exerciseable. A typical vesting schedule is 25% per year on the anniversary of the grant date. That means that after one year up to 25% percent of the options granted may be exercised; after two years, up to 50%; after three years, up to 75%; after four years, all of the grant.
As another example, cliff vesting describes a vesting schedule in which all options become exerciseable on a single date.
Performance-based vesting and reverse vesting are other examples of vesting schedules.