Return period কী?

The return period $T$ of a flood is the average interval, in years, between years whose annual peak equals or exceeds it. A flood of return period $T$, a $T$-year flood, has a probability $1/T$ of being equalled or exceeded in any one year.

Let events of a given kind — a flood above a discharge, a ground motion above a level, an earthquake above a magnitude — occur at a mean annual rate $\lambda$. Their return period is $T = 1/\lambda$: the mean interval between two of them, and the inverse of their annual rate, not a timetable.