Man-In-The-Middle Attack definition in Computer Security terms:
Definition(s): An attack on the authentication protocol run in which the Attacker positions himself or herself in between the Claimant and Verifier so that he can intercept and alter data traveling between them.
Source(s): NIST SP 800-63-2
A form of active wiretapping attack in which the attacker intercepts and selectively modifies communicated data to masquerade as one or more of the entities involved in a communication association.
Source(s): CNSSI 4009-2015 (IETF RFC 4949 Ver 2)
reference: CSRC Glossary