The concept of session is a fundamental one in web analytics, where the number of pageviews per session or the duration of a session are often used as KPIs. Incidentally, deduplication aims to identify the different sessions opened by a single user across various devices in order to measure not the number of devices but the actual number of users.
Session
A browsing session is the period during which a user is connected to a website or a mobile app. It starts when the user reaches the website (interpretations of this tend to differ depending on the measuring tool) and ends when he leaves it (this is recorded after a certain period of inactivity, the standard duration being 30 minutes).
