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.
Sources:
CNSSI 4009-2015
from
IETF RFC 4949 Ver 2
An attack in which an attacker is positioned between two communicating parties in order to intercept and/or alter data traveling between them. In the context of authentication, the attacker would be positioned between claimant and verifier, between registrant and CSP during enrollment, or between subscriber and CSP during authenticator binding.
Sources:
NIST SP 1800-21B
under Person (Man)-in-the-Middle Attack
An attack where the adversary positions himself in between the user and the system so that he can intercept and alter data traveling between them.
Sources:
NISTIR 7711
under Man-In-The-Middle