What is the term for interference from a signal at twice the IF frequency from the desired signal?

An 'image response' occurs when a receiver picks up an unwanted signal that is spaced exactly twice the intermediate frequency (IF) away from the desired signal. Because a mixer outputs both the sum and the difference of the local oscillator (LO) and any incoming signal, an unwanted signal on the 'wrong side' of the LO will also produce the exact same difference frequency as your desired signal. This mirror-image signal is called the image frequency. Good receiver design utilizes RF filters before the mixer to block these image frequencies from ever entering the mixer, preventing image response interference.