Which term describes the ability of a receiver to discriminate between multiple signals?
Why Selectivity is a receiver's ability to separate a desired signal from others nearby in frequency, and it is set mainly by the bandwidth and shape of the receiver's filters (IF filters, crystal or DSP filters). A narrow CW filter of a few hundred hertz gives high selectivity because it rejects signals just a kilohertz away, while a wide FM filter passes much more of the band. Poor selectivity shows up as adjacent signals bleeding into the one you are trying to copy.
Watch out Sensitivity is the other common receiver spec, but it describes how weak a signal the receiver can detect, not how well it separates signals that are close together in frequency.
Selectivity = Separating signals; Sensitivity = Seeing weak Signals.