Die Kennzahl Visitor, zu Deutsch der Besucher, bezeichnet die Anzahl der Besucher einer Website im Betrachtungszeitraum. Jeder eindeutig erkannte Besucher wird dabei nur einmal gezählt, unabhängig wie oft er die Website erneut besucht. Die Metrik Visitor basiert meist auf einem Cookie. Löscht ein Benutzer seine Cookies, beginnt die Visitor-Zählung in jedem Fall neu. Manchmal benutzen Tool-Anbieter auch den Namen Unique Visitor. Das ist aber oft falsch, da nicht ein einzelner Besucher identifiziert wird, sondern nur der Browser des Benutzers, unabhängig davon wie viele Personen diesen benutzen (Firmen, Bibliotheken, Internet Cafes).

Synonyme: Visitor, Unique Visitor, Besucher, User, Unique Audience

Im Vergleich1:

IVW: Die IVW und die AGOF bemühen sich derzeit um eine aktuelle Definition (siehe dazu den Artikel von Netzwertig).
Juli 2008 : The unique user states the number of persons that have been exposed to a particular advertising medium (website) or booking unit during a given period of time. The unique user serves as the basis for calculating coverage data and structural data of online advertisements and for the calculation of key factors of media planning such as weekly consumption, monthly consumption and contact establishment. Within the data set for the internet facts, each line represents one unique user. The Unique User is neither a visit nor a client, but it represents a real person.
WAA: The number of inferred individual people (filtered for spiders and robots), within a designated reporting timeframe, with activity consisting of one or more visits to a site. Each individual is counted only once in the unique visitor measure for the reporting period.
IAB: Where a web server stores a small piece of information with a browser which uniquely identifies that browser. While cookies only identify unique computers – as opposed to individuals – the inactivity constrain on the calculation of visits, i.e. 30 minutes, should make it relatively safe to use cookies to determine the page requests associated with one. One caveat is caching: reportedly, some online services are caching the cookies, thus requesting pages for multiple visitors. Another occurs when you count visits just by cookies, you will end up with a batch of pages for visitors without cookies. You must use one of the other methods to estimate.the numbers of visitors that created this batch of requests.
Comscore: An individual that visited any content of a website, a category, a channel, or an application during the reporting period – typically a calendar month. An individual may be classified as a user by accruing a page view, or by accessing rich media content (eg, a video or widget).
Nielsen: Unique Audience: The total number of unique persons that have visited a website or used an application at least once in the specified reporting period. Persons visiting the same website or using the same application more than one time in the reporting period are only counted once.
Unduplicated Audience: The total number of unique persons that have visited any one of the websites selected.

1 Die jeweils vollständige Definition ist auf der Website der Organisationen nachzulesen (die Web-Links finden Sie hier).