pinup wiki:
A style, usually featuring well-developed women, which accentuates sexy with situations where the women is either innocent, unaware of her body, or confident.
What? This definition dates back to 2008, yet it sounds like it might apply to about two-thirds of the site. The question is which two thirds. It's no better than cute or hot, IMHO.
This tag might be useful if it was changed to denote something like "Pictures that are (primarily) a full-body view of a single character, in a sexually provocative pose.". Still, the first part of that definition is basically solo full_body, and 'provocative' is quite subjective.