Also known as "return period" or "recurrence interval", flood frequency is a hydrologic terminology, which means the average recurrence interval of floods over an extended period of time. In other words, it denotes that the flood equal to or larger than a particular size happens once in a period of time. Thus, it is a term related to probability but does not contain any meaning of time cycle. The term is typically used for river protection standards (indicating the sizes of various floods). Take the comparison between “2-year flood frequency" and "100-year flood frequency" as an example. The “2-year flood” happens more often, indicating smaller amounts of flooding, while the “100-year flood” happens much less, indicating a much larger water volume. Nevertheless, the occurrence of a flood in 100-year flood frequency this year does not mean it will only happen again 100 years later. Flood of the same size may happen two years in a row.