A vesting schedule in which the percentage of stock options or restricted stock/RSUs that vest in each tranche increases over the course of the vesting period. For example, a grant may vest 10% in the first year and then 20%, 30%, and 40% over the subsequent three years. This structure can encourage employee retention, as you need to work longer at the company for the bulk of the grant to vest. However, in tight labor markets where they can easily switch jobs, employees prefer front-loaded vesting, i.e. more of the grant vests earlier.