What's The REAL Definition Of A Unique

Got a bit of a debate going here because I have a site that is showing wildly varied results for traffic between different trackers.

The three we are watching are:

extreme paid version

awstats

webalizer

Extreme shows 70k uniques for this month while awstats is showing 16,555 then to make things even more confusing webalizer is showing 22564.

So that leads us to the question, what exactly is a unique and is there really any precise definition of how a unique should be counted. is it:

1. one unique IP visit for the whole site

2. one unique IP visit per page on the site

then on top of that we have to factor in the time frame so now is it:

1. one unique IP visit for the whole site per day

1. one unique IP visit for the whole site per month

3. one unique IP visit per page per day

4. one unique IP visit per page per month

Now some may say the above is being picky BUT how many site owners do you hear saying "I get 100,000 uniques per month". This comment alone begs for an answer to this question. I don't think I have ever heard someone come back at a comment like that with "Really, How do you count your uniques?"

With all the trackers out there and all the companies that are relying on accurate statistics to sell advertising and / or value the site this is one of THE grayest areas around ......

So again, the question is:

What the heck is the definition of a unique and for that fact is there one?