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).
IMAP is a newer alternative to the POP protocol. Compared to POP, IMAP offers more intelligent handling of sent mail and organizing mail by folders, and also better handling of email accounts that are checked from multiple clients, such as checking your email from both your PC and a mobile device.
There are also proprietary alternatives to POP and IMAP, such as Outlook/Exchange and BlackBerry.