A return period also known as a recurrence interval is an estimate of the likelihood of an event, such as an earthquake, flood or a river discharge flow to occur. It is a statistical measurement typically based on historic data denoting the average recurrence interval over an extended period of time, and is usually used for risk analysis (e.g. to decide whether a project should be allowed to go forward in a zone of a certain risk, or to design structures to withstand an event with a certain return period.
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....This does not mean that a 100 year flood will happen regularly every 100 years, or only once in 100 years. Despite the connotations of the name "return period"in any given 100 year period, a 100 year event may occur once, twice, more, or not at all, ....