The FEMA 100 year flood zone explained.
I use the term “100-year flood zone daily for elevation certificates, LOMA’s, and explaining flood maps. But it doesn't mean what you might think. It means there is a 1% chance you will see a flood like the one on the FEMA flood map each and every year. Since 1% is also "1 out of 100", the term "100-year flood" was adopted because that's easier to talk about than rattling off a bunch of statistics.
FEMA sometimes shows a 500-year flood on their maps and that is technically the 0.2% annual chance flood. Try saying "zero point two percent annual chance flood zone" two dozen times a day and you can see why we use the short version.