A Visitor (or rather a Unique User) is an individual user who has accessed a website during a specific period of time (e.g. day, week, month). A visitor is identified using cookies, IP addresses, or other identifiers to ensure that the same person visiting the site multiple times is not counted as a new user each time.
Conceptually, a visitor is the basic unit of measurement for the audience and reach of a website. The number of unique visitors is a key indicator of the effectiveness of all traffic-driving channels (SEO, PPC, social media).
In an analytical context, unique visitors are distinguished from Sessions, which represent the total number of interactions, and Pageviews, which represent the total number of page views. Organic Search Traffic is measured precisely by the number of unique visitors that come from the SERP.