Session ID

A session ID is the identifier that groups all events of one continuous visit under a single session and changes when that visit ends.

A session ID is the shortest-lived identifier in the identity hierarchy: it sits beneath the user ID and device ID and spans just one visit. The most important point is that a session boundary is a convention, not a natural fact; a session usually closes after a period of inactivity and the next visit gets a fresh one, and that threshold determines how far 'one session' stretches. A common mistake is reading session-based metrics like session duration without regard to this rule: if the inactivity threshold is 30 minutes, a user who reads a long article and then returns may be counted as two sessions. So a session ID is a good unit for measuring 'one visit', but for measuring 'one person over time' you must turn to the user ID.