Spoofing

Involves—
1) the ability to receive a message by masquerading as the legitimate receiving destination, or
2) masquerading as the sending machine and sending a message to a destination.
SOURCE: FIPS 191
1. Faking the sending address of a transmission to gain illegal entry into a secure system. Impersonating, masquerading, piggybacking, and mimicking are forms of spoofing.
2. The deliberate inducement of a user or resource to take incorrect action.
SOURCE: CNSSI-4009