A storm that, on the average, is equalled or exceeded once every 100 years. In any one year the chance of it being exceeded is 1/100 or 0.01. This storm event is often tied to duration of storm. For example a storm may be classified as a '100 yr' storm for its precipitation over 24 hours but may not have been a 100 yr storm for its peak 1 hour precipitation. And yes you can have 2 or more 100-yr storms in less than a 100-year period.
