What is Session (visit)?
A session, or visit, is a sequence of actions by one visitor with no long gap between them. It ends after a period of inactivity, conventionally 30 minutes. That threshold is an industry convention inherited from early log analysis rather than anything observed about human behaviour, and changing it changes every session-based metric you have.
Where the 30 minutes came from
It was a reasonable guess made when analytics meant parsing server logs, and it stuck because everyone needed to agree on something. It is not derived from data about attention spans, and it produces obvious artefacts: a reader who leaves a tab open through lunch and returns starts a “new visit” at the same page they left.
Some tools also start a new session when the campaign source changes mid-visit, so a visitor who clicks your ad, leaves, and returns from an email within the window may be two sessions or one depending on the tool. Neither is wrong. They are different definitions wearing the same word.
Sessions, visitors, pageviews
pageviews ≥ sessions ≥ visitorsOne visitor can have many sessions; one session can have many pageviews. Ratios between them carry real signal — pageviews per session is depth, sessions per visitor is return frequency — as long as the period and the tool are held constant.
What a session is good for
It is the natural unit for anything about one continuous experience: where a visit started, what path it took, whether it ended in a conversion, whether it bounced. Bounce rate and visit duration are both session-scoped by definition, and neither means anything without it.
It is a poor unit for audience size, where you want visitors, and a poor unit for content popularity, where you want pageviews.
Cookie-free sessions
Sessions do not require a cookie. A session identifier can be derived per request from a rotating salted hash of the visit’s characteristics, which is enough to group activity within the window without leaving anything on the visitor’s device — and without giving anyone the ability to follow that person next week.