In U.S. weather forecasting, POP is the probability of exceedance that more than 1/100th of an inch of precipitation will fall in a single spot, averaged over the forecast area.[1] For instance, if there is a 100% probability of rain covering one side of a city, and a 0% probability of rain on the other side of the city, the POP for the city would be 50%. A 50% chance of a rainstorm covering the entire city would also lead to a POP of 50%.
Note that the POP measure is meaningless unless it is associated with a period of time. U.S. forecasts commonly use POP defined over 12-hour periods (POP12), though 6-hour periods (POP6) and other measures are also published.