Also called: sequence risk
The risk that poor market returns arrive early in retirement. Two retirees with the same average return can end up very differently if one suffers the losses while withdrawals are just beginning.
Withdrawals taken after a decline sell more shares to raise the same dollars, and those shares are not there for the recovery. The order of returns does not matter while no money is going in or out; it matters once you are withdrawing.
The years just before and after retirement carry the most exposure, because the portfolio is at its largest relative to the spending it must support.