100-year floodplain

'100-year floodplain' is the everyday name for what FEMA's flood maps formally call the Special Flood Hazard Area: land with at least a one-percent chance of flooding in any given year. The name misleads more people than it informs — it names an annual probability, not a hundred-year cycle, and a lot in the 100-year floodplain can flood in consecutive years or not at all for decades. New York's flood-disclosure forms use the plain name directly: both the amended Property Condition Disclosure Statement and the residential-lease flood notice ask whether a property sits in the 100-year floodplain.

The practical consequences ride on the formal designation, not the nickname: mandatory flood insurance on federally backed mortgages, and elevation and construction standards for new or substantially improved buildings. A property can sit outside the 100-year floodplain and still carry real flood risk — New York City's stormwater flooding routinely occurs outside every mapped FEMA zone.