MIM - Also - MITM - Man In the Middle attacks are a type of security breach or attack on information communications. This attack assumes adversaries can intercept or even modify the traffic passing between two communicating parties. MIM attacks succeed if the systems can't distinguish communications with an intended recipient from those with the intervening attacker. This is the main threat type the SSL protocol was designed to defend against.

Perspective: In practice, the threat from MIM attacks is rather small. In fact, to date there has never been a single instance of an MIM interception of a credit card sent over an HTTP link.

See http://www.iang.org/ssl/ for more information and perspective on this issue.