What is the ability of a filter circuit to reduce unwanted frequencies known as?

The ability of a filter circuit to reduce unwanted frequencies is known as attenuation. This term refers specifically to the reduction in the amplitude of a signal as it passes through the filter, effectively suppressing frequencies that are not desired while allowing others to pass through with minimal loss.

Filters are designed to target specific frequency ranges, either allowing certain frequencies to pass (passband) or rejecting others (stopband). The amount of signal voltage reduction represents how effectively the filter is attenuating unwanted frequencies. Higher attenuation indicates that the filter is more effective at reducing these frequencies.

Other terms like isolation, signal processing, and discrimination relate to filtering but do not directly refer to the action of reducing unwanted frequencies. Isolation often pertains to the separation of signals, signal processing involves manipulating signals for various purposes, and discrimination refers to distinguishing between different signals but doesn't specifically imply the reduction of amplitude. Thus, attenuation is the most accurate term in this context.