Final answer:
The return period/recurrence interval is a probability measure in Mathematics, specifically related to flood recurrence intervals.
Step-by-step explanation:
The subject of this question is Mathematics. Specifically, it is related to the concept of flood recurrence intervals.
A return period, also known as a recurrence interval, represents the probability of a specific event occurring in a given time frame. It is not a fixed value but rather a measure of the likelihood of an event happening.
For example, if we calculate a recurrence interval of 5 for a flood event, it means there is a 1-in-5 chance of that particular flood magnitude occurring in any given year.
However, it does not mean that a flood of that magnitude will occur exactly every 5 years. There could be multiple occurrences within a short span or long gaps between events of the same magnitude.