(1) Post Office Protocol
e.g. POP3 is the protocol used by many e-mail clients to retrieve a person's e-mail from a mail server. POP3 is one of the most popular services on the Internet, and one of the favorite services hackers try to break into.
(2) Point Of Presence
a web site, that refers to your ISP, that can offer you an access point to the Internet.
(3) Proof of Possession
see X.509 certification request ( RFC2511 )