Post Office Protocol, version 3.
The Post Office Protocol (POP) is an application-layer Internet standard protocol used by local e-mail clients to retrieve e-mail from a remote server over a TCP/IP connection.
The acronym POP is usually accompanied by a version number. The first (POP1) was introduced in 1984, the second (POP2) in 1985 and the last (POP3) in 1988. Nowadays, POP stands for POP3.
See wiki for refence: Post Office Protocol