Acronym: Internet Message Access Protocol
A protocol used to receive E-Mail from a server, allowing the user to read mail locally, instead of logging in to the server via Telnet/SSH using an E-Mail client such as Pine to read mail. The latest version of the protocol is version 4. Port 143 is assigned by IANA for IMAP4. Some servers support IMAP4 over SSL. Port 993 is assigned by IANA for IMAPS (IMAP over TLS/SSL).
Although POP3 is an easier protocol, is easier to implement for programmers, and is more widely used IMAP provides some advantages over POP3. When using IMAP, it at first only downloads the header of one file. One can then decide what to do with the message. Ie. mark as spam, virus, delete, read, move. IMAP also supports multiple maps, serverside, thus making it possible to organize E-Mail serverside. POP3 doesn't support this. Some Webmail services do support this.
Originally developed at Stanford University in 1986 (history).