A visitor is technically defined as a unique computer's browser viewing your site. Unique visitors are tracked over long periods of time. The tracking is accomplished with an aggregate of the following unique variables: Timestamp (millisecond), IP address (reverse DNS lookup), User Agent, Cookie and/or Registration ID. Each visitor is a unique person, counted once.
WAA Definition: 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.