A man-in-the-middle (MITM) attack is an active attack where the attacker is able to interpose himself between the sender and receiver. The attacker may monitor and/or modify some or all of the messages sent between the two endpoints.
Man-in-the-middle (MITM) is an attack where a user gets between the sender and receiver of information and sniffs any information being sent. In some cases, users may be sending unencrypted data, which means the MITM can obtain any unencrypted information. In other cases, a user may be able to obtain information from the attack, but have to decrypt the information before it can be read.
Another more practical example of a man-in-the-middle attack is one in which the attacker intercepts messages in a public key exchange and then retransmits them, substituting his own public key for the requested one, so that the two original parties still appear to be communicating with each other.