When measuring the characteristics of a small-signal amplifier, say for a radio receiver, one might be concerned with its "Noise..."?

"Noise figure" is one critical parameter for determining the "quality" of a low-noise, small-signal, amplifier, and perhaps more importantly, the sensitivity of the overall receiver system. It is especially important for VHF (very high frequency) - and higher frequency - designs where most of the undesired "noise" (noise that masks or overpowers the desired signals) is generated internal to the electronics itself. In HF (high frequency) applications, atmospheric noise plays a much larger role in determining overall useful sensitivity, so a circuit's "noise figure" is less important.