The collectivity of people, separated on the basis of mutually experienced events, that occurred at a particular time or period.

The purpose of separation is the observation and analysis of selected demographic and social facts in the life of members of distinct communities, starting with their common events such as year of birth, year of marriage, a group of people starting their education in a particular year etc. Cohorts are used in comparative studies, for example between the different generations, or examine the same age cohort after a certain time.