A vesting cliff is a structural mechanism in equity compensation plans. It dictates that an employee earns 0% of their stock grant strictly until a massive, singular date is reached—the 'cliff.'
The most standard structure for startup equity is a '4-year vest with a 1-year cliff.' If you leave the company on day 364, you walk away with nothing. On day 365, you instantly vest 25% of the total 4-year grant. Thereafter, the remaining 75% typically vests monthly or quarterly.
This acts as an incredibly powerful retention mechanism, heavily disincentivizing engineers and executives from jumping ship before proving their value for a full year.