Glossary: % Comp UV

% Comp UV: (Or, Percent Composition of Unique Visitors) The percentage of a site’s visitors that are in your target demo.

This is why reach ain’t everything. If you have a small demo especially, you might not want to blow a ton of money on big sites with huge reach to hit them. If you try to figure out smaller sites whose visitors are highly-composed of your demo, you can probably buy on a bunch of those sites for cheaper, and still end up getting the same (or close to it) reach.