What term specifies a filter’s maximum ability to reject signals outside its passband?

A filter's passband is the range of frequencies it allows to pass through, while frequencies outside this passband are attenuated. The absolute maximum amount of attenuation the filter can apply to these unwanted out-of-band signals is called its 'ultimate rejection.' Distractors like 'notch depth' apply specifically to notch filters (which reject a very narrow band of frequencies), while 'rolloff' refers to how steeply the filter transitions from passing to rejecting signals. 'Insertion loss' is the signal power lost within the passband simply by inserting the filter into the circuit.