Quick Question: Are absolute unique visitors and visitors the same thing?

[Tristan Lee]

For example, if I have a total of 100 unique visitors for a span of one month, does that mean that every single of one these people have different ISP addresses?

Or is it possible that "unique" just means how many unique visitors I get in a day and visitors who come in one day can also be the same people who come the very next day, making them not so unique?

Can somebody clear this up for me?

Thank you.