A unique visitor is a count of the distinct individuals who access a website within a specified reporting period, with each person counted only once regardless of how many sessions or pageviews they generate in that window.
Because 'distinct individual' is hard to observe directly, analytics tools approximate it using some identifier — a cookie, a device fingerprint, or a hashed combination of IP address and user agent in cookieless setups — and that choice directly affects the count. A visitor who clears cookies, switches devices, or browses privately can be counted more than once even though they're the same person, a known limitation of the metric rather than a bug in any given tool.
If the same person visits a site from their laptop in the morning and their phone in the evening, most tracking methods count that as two unique visitors, since there's no shared identifier linking the two devices.
Different tools use different identification methods — persistent cookies, session-based identifiers, or hashed non-cookie signals — so the same real traffic can produce different unique visitor counts depending on how each tool defines 'unique'.
They're generally treated as synonyms in most analytics platforms, both referring to the count of distinct people rather than distinct sessions or pageviews.
Yes — cookieless tools typically estimate uniqueness using a rotating, non-persistent hash of signals like IP address and user agent, trading some precision for not storing identifiable data long-term.