Discount rate
Also known as: Required rate of return, Hurdle rate
The discount rate is the return an investor would reasonably demand for waiting on a future cash flow instead of having that cash today. In a discounted cash flow model, every future year's projected cash gets divided down using this rate, since a dollar received later is worth less than a dollar in hand now.
The rate is doing two jobs at once: accounting for time, money available today can be put to work immediately, and accounting for risk, the less certain a cash flow is, the more return an investor demands for holding it. A stable, predictable blue chip might reasonably use a discount rate close to what a safe government bond currently yields. A small, unproven, or speculative business might reasonably use 15% or more.
Nudging the discount rate by even a percentage point or two can move a DCF's total estimate by a wide margin, especially for cash flows further out in the forecast. It is usually the single most sensitive assumption in the whole model.