A session is a group of user interactions with a website bundled into a single unit, from arrival to departure. For example, if a user visits a site, views 3 pages, and leaves, that counts as one session.
Session boundaries vary by analytics tool. In Google Analytics 4 (GA4), a session ends after 30 minutes of inactivity (timeout), and this 30-minute window can be adjusted in the admin settings. The older Universal Analytics also split sessions at midnight (when the date changed) or when the traffic source changed, but GA4 has removed those rules and, in principle, determines sessions solely by inactivity.
Understanding the difference between sessions and pageviews (PV) is important. If one session includes 5 page views, the session count is 1 and PV is 5. The distinction from unique users (UU) also matters: if the same user visits once in the morning and once in the afternoon, UU is 1 but session count is 2.
In short URL click analytics, the session concept is crucial. When the same user clicks a short URL multiple times, raw click count and unique click count differ. To accurately measure marketing effectiveness, compare unique clicks against session data to understand how many actual users reached the destination.
Key session-related metrics include pages per session (engagement depth), average session duration (engagement quality), and bounce rate (single-page sessions). GA4 introduced "engaged sessions" - sessions that last over 10 seconds, include 2+ page views, or trigger a conversion event.