1. Post Office Protocol
An Internet protocol used to download messages from an email server to an email client (usually email software running on a PC or mobile device).
POP3 is the most common email-downloading protocol. IMAP is a newer alternative to POP, and there are proprietary protocols as well, such as Outlook/Exchange and BlackBerry.
2. An abbreviation for "population". To say that a radio network or license covers "10 million POPs" means that 10 million people live within the geographic area covered by that network or license.