Traffic
Visitor
Also called user, client.
Visitor is the identifier a tool uses for someone who sent a hit. It stands in for a person but is not one.
How it is measured
The identifier may be a first-party cookie, a hash of request signals, or a login ID. The count of visitors is the number of distinct identifiers inside the report window.
Tarsier derives its visitor signal without cookies and keeps nothing across days, which is a deliberate privacy choice. The cost is that it never follows a person from Tuesday to Friday.
Worked example
A family shares one tablet at a travel-booking site. Three people search for flights from it in one evening, and the tool sees one visitor. Later, one of them checks the same trip on a work laptop and appears as a second.
The agency reads two visitors where there are three people on two devices, and the reality is still a family planning one holiday.
How it differs
A visitor is the identifier. A unique visitor is the de-duplicated count of those identifiers. A session is one sitting of a visitor. A user ID is an identifier your site supplies after login.
Common errors
Treating a visitor as a person. Ignoring shared devices. Counting a laptop and a phone as one. Forgetting that clearing storage mints a new one. Leaving bots in the total.
In practice
Treat visitor counts as estimates of audience size. If your product has logins, send a user ID and use it when you need people.