POP
Definition:
POP stands for Post Office Protocol. The protocol is used between mail clients (the software you use to check your email) and mail servers (the computers that store your email) to negotiate the sending and receiving of email. POP is often seen listed as POP3 which represents version 3 of the protocol.
The POP protocol is similar to the IMAP protocol except that with POP, the mail store (all historical messages) is kept on the user's computer making it difficult to access the full history of email messages on any other computer.