Man-in-the-middle Attack (MitM)

An attack on the authentication protocol run in which the Attacker positions himself in between the Claimant and Verifier so that he can intercept and alter data traveling between them.
SOURCE: SP 800-63

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: CNSSI-4009