The period at the start of a vesting schedule during which you earn nothing at all — leave before it, and you keep zero.
The standard is four-year vesting with a one-year cliff: nothing until month twelve, then a quarter lands at once, then monthly.
It exists for the case where a cofounder leaves in month three. Without a cliff they keep three months' worth forever; with one, they keep nothing and the company stays fundable. It is not a statement about trust — it is what makes an even split survivable.
It should be mutual. A cofounder who objects to symmetric vesting is telling you something worth hearing early.
Project types where this term stops being vocabulary and starts being a decision you have to make.