The Probable Maximum Flood (PMF) is defined as the largest flood that is considered reasonably possible to occur at a specific location, based on the most severe combination of meteorological and hydrologic conditions. It represents an extremely rare but physically possible flood event, and is commonly used for dam safety analyses and critical infrastructure design, especially when failure would have significant consequences.

Hydrologic systems can be affected by extreme events such as heavy storms, floods, and droughts. These events vary in severity, and generally, the more severe an event is, the less often it occurs. This relationship—between how extreme an event is and how frequently it happens—is the focus of frequency analysis in hydrology.