Whats considered a Unique Visitor?

Unique visitors are people who have never visited before.

Or use www.statcounter.com. This is how statcounter determines a unique visitor:

Maximum Visit Length
Used to calculate your unique and returning visitors from a cookie. If this amount of time or more has elapsed since a visitor last visited a page on your website, then that visitor is considered unique. We recommend setting it between 1 and 6 hours. We don't recommend setting it to 24 hours and above.