Exponential decay is when a numerical value reduces by a fixed proportion in each unit of time (or some other variable). For example, if the amount of water reduces by a third every day. When this happens the value at time t is Kt where K is a constant less than 1, or alternatively e–λt where λ = ln(K), the natural logarithm of K. Exponential decay reduces faster than any positive power of 1/t.
Used on Chap. 14: page 334