Vesting
Also called Cliff and Four-Year Vesting
Earning your equity over time instead of all at once. The standard is four years with a one-year cliff: nothing vests until you have been around a year, then it accrues monthly.
Why it matters
Vesting protects the company and the remaining founders if someone leaves early. Founders should vest too, investors expect it, and a co-founder without vesting is a risk that can sink a company if the partnership breaks.