Extreme rainfall events
We define rain events in terms of the probability of reccurrence. For example, we call a certain rainfall the "10-year rain event," meaning that this amount of rain would only be expected to occur once every 10 years. We usually like to have at least 30 years of data before calculating recurrence intervals.

Note: Just because we call something the 100-year storm event does not mean that it will only occur every 100 years.  Sometimes they occur much more frequently.  Sort of like winning the lottery twice in a row when the odds of winning at all are quite low.  Unlikely, but possible.  

The 100-year storm also changes with the available data.  If we were to have several "100-year storm events" in the span of 10 years, we might need to go back and determine a new 100-year event that takes recent data into account.  This is especially important with respect to climate change.  In Boston, the 100-year flood event is projected to occur every 2-4 years by 2050 or so.  Obviously we then need to go back and calculate what a new 100-year event is. 

USGS Q&amp;A on flooding 

NE Aquarium Climate Change page