IMAP stands for Internet Message Access Protocol, a protocol for email retrieval. IMAP is an Application Layer Internet protocol that enables an email client, like Microsoft Outlook for example, to access email on a mail server.
Email clients that use IMAP will generally leave messages on the server until the user deletes them, which along with other elements of IMAP allow multiple email clients to access and manage the same mailbox. While most email clients support both IMAP and the Post Office Protocol (POP) to retrieve email messages, but more email services support POP than IMAP.
Some of the advantages of IMAP over POP are that it allows for both connected and disconnected modes of operation, permits multiple clients to connect to the same mailbox simultaneously, and mechanisms that allow the email client to retrieve just the text portion of a message without any attached files, or to stream content as it continues to be fetched. IMAP4 also allows the client to request a search by the server for messages that meet a broad range of criteria, which avoids the client having to download every message in a mailbox in order to perform such searches on the client-side.
IMAP was created in 1986 by programmer Mark Crispin as a remote mailbox protocol, and was originally called Interim Mail Access Protocol, which was later changed to Interactive Mail Access Protocol (IMAP2) when the updated protocol replaced the interim version in 1988.