Sessions

A session is a group of one user's interactions with your site or app within a window of activity.

A session starts when someone arrives and runs until they go idle. By default GA4 closes it after 30 minutes of inactivity, then opens a fresh one if they come back later. One person can rack up several sessions in a day, and unlike old Universal Analytics, a new session in GA4 does not restart just because the clock ticks past midnight or a new campaign tag appears mid-visit.

Someone reads two of your blog posts in the morning, closes the tab, and returns that evening to read a third. That is two sessions from one user. If instead they read all three back to back with no 30-minute gap, it stays a single session.

Sessions in GA4 are counted differently than in Universal Analytics, so do not expect the numbers to line up. GA4 estimates session counts using the session_start event rather than the old rules, and it stopped resetting sessions on campaign changes. Migrated properties almost always show fewer sessions than the old ones did. That is a definition change, not lost traffic.

One visit, one window of activity, one session.