What term specifies a filter’s attenuation inside its passband?

'Insertion loss' is the term used to describe the reduction of signal power (attenuation) that results from inserting a component, such as a filter, into a circuit. Even within a filter's passband—the range of frequencies it is specifically designed to let through—there is always a small amount of signal loss due to the physical resistance and imperfections of the components. Other terms listed are related but distinct: 'Return loss' relates to impedance mismatch and signal reflections, 'Q' is the quality factor determining the filter's bandwidth, and 'ultimate rejection' refers to the attenuation of signals outside the passband. Therefore, insertion loss is the correct measure for in-band attenuation.