Definition of a visitor
In web analytics a visitor is an unique user visiting a site.
Another term describing visitors is unique impressions.
There are several methods of tracking visitors, but the standard method is by cookies.
It is important to keep in mind that an unique user is not the same as an unique person.
With the advent of smartphones, tablets, smart TV's, office computers, home computers, laptops etc. a person is likely to be counted as a unique user several times over the course of a day.
Many users also delete their browser history and cookies regularly, and some use some kind of incognito mode to prevent logging of activity.
I also recommended using the CPM calculator to make sure you get your numbers right.