What is a unique visitor?
The unique user metric counts individual users to the website over a given period of time. This metric measures the user regardless of their number of visits or whether they were a first-time visitor to the web page. Every time a user visits a web page, their IP address assigns them a unique identifier that google analytics can then track to understand the number of times they visit the site, their page views, and other user behavior.
When a unique visitor returns to the site as a returning visitor, they are tracked under the same attribution identifier that they were assigned on their first visit. The number of unique visitors is not the same as the total number of site visits but instead represents each person.