How Unique Is A Unique Visitor?

It depends on what you're using the data for, but in practice, it tends not to matter. Web Analytics is more concerned with trends over time and with comparisons between different areas (sections of the site, conversion funnels, traffic sources, etc). You're concerned with which areas are performing better or worse in order to optimize paths or allocate resources for improvements, and the absolute performance simply doesn't affect the relative performance. In situations like this, consistency is far more important than accuracy.

The one major thing that it bungles up is return visits. If people see your site while surfing at work, and it's intriguing enough to come back and give it a deeper look at home, you'll never see the connection.

There are a few analytics tools that merge users across browsers. Unica's NetInsight, for example, can coalesce multiple cookied browsers into a single user if that user logs in with the same account on multiple browsers. The fact that it only works on authenticated users limits its applicability.

Recently submitted: http://news.ycombinator.com/item?id=1133928

Comments already there.

This search found it trivially: http://searchyc.com/unique+visitor?sort=by_date

This is interesting because there are a number of derived numbers that may be very different than you think. If you for instance think the rate of uniques that turn into paying customers is 2.5% it may actually be 5% which is a very different number.