MITM. An intermediate party acting as a proxy for clients on either side. Also a method of attacking a secure transmission, whereby the MITM intercepts and forwards messages without either party knowing it. This gives the potential to eavesdrop, change the message or collect private information. Applies to any message, encrypted or not. Mutual authentication is one form of protection. Trusted agents (eg a Root Certificate Authority) are another.

Source: http://identityaccessman.blogspot.com/2006/08/identity-dictionary.html