What is the term for interference from a signal at twice the IF frequency from the desired signal?
Why In a superheterodyne receiver the mixer combines the incoming signal with a local oscillator, and any signal that differs from the LO by the IF will pass through the IF filter. Besides the desired frequency there is a second one, offset from the desired signal by exactly twice the IF, that also produces the same IF output, so it gets heard right along with the wanted station. That unwanted response is called the image, and front-end selectivity (or a higher first IF) is what rejects it.
Watch out Quadrature response sounds technical but refers to signals 90 degrees apart in phase, as used in some detectors, not to an off-frequency signal sneaking through the mixer; 'mixer interference' and 'intermediate interference' are not standard terms.
Image = 2 x IF away from the desired signal. Picture a mirror image on the far side of the LO.