I see 12 questions using this tag.
25% of the questions are related to pop-ups.
75% are related to the Post Office Protocol.
So (at the time I'm making this question) we really have two separate topics sharing the same tag. I believe that is not ideal/desired.
pop as referring to the mail protocol isn't a bad tag. It definitely shouldn't mean "Post Office Protocol" and "pop-ups" at the same time, though. There is already a much more widely-used pop3 (>170 questions). I don't imagine there will be many if any questions about earlier versions of POP, so there's nothing wrong with moving pop-tagged POP3 questions into pop3. If there are questions about previous versions, they can keep their current tag.
Pop-up questions don't need a special tag in my opinion; the program/browser tag ought to cover it for categorization/discoverability purposes. Nevertheless, we do have popups.
Since POP reached version 3 before Internet popularity became mainstream, and then stayed at that version number, nearly all POP users use POP3.
One solution: split into two tags by renaming POP to POP3, and make a "pop-ups" tag. However, I haven't been very involved in tag maintenance. Perhaps experienced people would prefer another solution, such as slapping an E-Mail tag onto all POP3-related questions, and not even bother having a POP3 tag for the measly number of just 9 questions over roughly 5 years.