(1) POP is short for Post Office Protocol, a protocol used to retrieve e-mail from a mail server. Most e-mail applications (sometimes called an e-mail client) use the POP protocol, although some can use the newer IMAP (Internet Message Access Protocol).
(2) Pop is short for point of presence, an access point to the Internet. ISPs have typically multiple POPs. A point of presence is a physical location, either part of the facilities of a telecommunications provider that the ISP rents or a separate location from the telecommunications provider, that houses servers, routers, ATM switches and digital/analog call aggregators.
(3) Pop is short for Programmed Operator (POP), a pseudo-opcode in a virtual machine language executed by an interpretive program. The Programmed Operator instructions provide the ability to define an instruction set for efficient encoding by calling subprograms into primary memory.
(4) POP is short for picture-outside-picture POP is a feature found on some televisions that allows the user to divide the screen into two same-size pictures, enabling you to view a second program. Compare with picture-in-picture (PIP).