The phrase "return period" refers to the amount of time it takes for a certain event, like a flood or earthquake, to occur again on average. It helps us understand the probability of something happening again in the future. Full definition
The relocation mask was defined as the set of areas with 3 or more meters of flood depth following an event with return period of 20 years, assuming no flood protections in place. (link.springer.com)
The manufacturer even guarantees a 90 day return period in case you're not quite happy with their oil. (geekwrapped.com)
The effect is minimal for average indices but is statistically significant in a subset of models for projected changes of 10 and 20 - year return periods. (pacificclimate.org)