Traffic
Session
Also called visit.
Session is a bundle of hits from one client treated as one sitting under a timeout rule. It is the unit behind bounce, duration and conversion rate.
How it is measured
The tool groups hits from the same client until it sees a gap longer than the timeout, commonly 30 minutes. Some tools also close the session at midnight or when the campaign source changes, which can cut one real sitting in two.
Tarsier's session id is random, kept in the tab's session storage, and never stored across days. Closing the tab ends the session and a new tab begins another, so a visit split across two tabs counts twice.
Worked example
A ticketing site watches a buyer open an event page at 19:02, leave to ask a friend, and come back at 19:41. With a 30-minute timeout the return starts a second session, so a single purchase is split across two. The first has no sale, the second does.
Raise the timeout to 60 minutes and the two merge into one session with a sale. The conversion rate rises, though the buyer behaved identically.
How it differs
A session groups hits. A visit is the same thing under an older word. A pageview is one load inside it, and a visitor is the client that can own many sessions. Counting sessions as people overstates audience whenever someone returns.
Common errors
Treating sessions as people. Not recording the timeout. Being surprised by a campaign-change split. Comparing figures from tools with different rules. Letting bot sittings pad the total.
In practice
Write the timeout and the midnight rule beside your dashboard and leave them alone. Use sessions for rates and per-visit measures, and visitors when the question is about audience size.