Summary
Annual exceedance probability, or AEP, is the clearest way to describe flood likelihood because it gives the chance of a flood event being equalled or exceeded in any one year. Return period language, such as 1 in 100 year flood, is common but easy to misunderstand. A 1 in 100 year event does not mean flooding happens only once every 100 years. It means there is about a 1% chance of that size of event being equalled or exceeded in any given year.
Definition
Annual exceedance probability is the annual chance that a flood of a given magnitude will be equalled or exceeded. A return period is the inverse expression of the same idea, for example 1% AEP is roughly a 1 in 100 annual chance and 0.5% AEP is roughly a 1 in 200 annual chance. Buyers should use AEP because it avoids the false impression that a so called 1 in 100 year flood cannot happen more than once in a short period.