A return period, also known as a recurrence interval or repeat interval, is the estimated average time between events such as floods to occur. It is a statistical measurement and is used usually for risk analysis. 

The theoretical return period between occurrences is the inverse of the average frequency of occurrence. For example, a 50-year flood has a 1/50 = 0.02 or 2% chance of being exceeded in any one year and a 100-year flood has a 0.01 or 1% chance of being exceeded in any one year.

A common misunderstanding is that a 1% event ("100-year flood") will happen regularly every 100 years or is likely to occur only once in a 100-year period. In any given 100-year period, a 1% event may occur once, twice, more, or not at all. In fact, there is approximately a 63.4% chance of one or more 1% events occurring in any 100-year period.