A group of individuals (users, customers, prospects) that exhibit the same or similar characteristics.
Cohorts are not limited to users with similar demographics (location, age, gender, etc.) & psychographics (interests, personas, etc.). Typically, users in a cohort also share certain experiences or events.
For example, users who subscribed to your website on the same date would be one cohort.
Google Analytics provides a Cohort Analysis report that allows you to view & analyse cohort data. This helps marketers understand the different cohorts with their audience, so they can personalise their marketing efforts accordingly.