A cliff refers to the minimum period of time that must elapse before a grant of equity compensation begins to vest. Consider the example of a common vesting schedule: 4 years with a 1-year cliff. In that scenario, a startup employee who's been granted equity wouldn't vest until they had worked for 1 year (i.e., the 1-year cliff) at the startup. If the employee left the startup at any time prior to the cliff, there would be no vesting.
