Sorry. It is going to be a boring one….
A cohort is a group of users who share a common characteristic that is identified in this report by an Analytics dimension. For example, all users with the same Acquisition Date belong to the same cohort. Cohort exploration allows you to explore the behavior of these groups over time on your app or site.
So, although it may be a boring word today. I do have high hopes that we will be seeing much more of the concept of cohorts as the deprecation of the third party cookie continues and the concept of new vs. returning visitor/user/customer is reshaped.