word: sensitivity. I think that's the better term, because it describes the input signal's effect as sound. So why does sensitivity matter so much? On line signals, it isn't decisive. But on a microphone, it's easy to show what it means. Try this: microphone in your hand, headphones on the desk output. Speak into the mic from a fixed distance and slowly open the gain. What happens? First, the level rises — we all agree on that. But to hear what's really happening, you have to listen more closely. As the gain rises, the mic's sensitivity — its field of perception — widens. The more gain, the wider the perception. What does that mean for us? As the field widens, the mic's sound spreads out — as if there were reverb inside it — because the mic is now picking up not just the main source but everything reflecting off the room; the result is an "effected" sound. At low gain, though — even if you bring the level back up with the fader or the master — the mic's sound is more "bony," fuller, and yet more transparent.
That's why working with as little gain as possible on mics is one of the most important details for making a mix easier. When many musicians ask for it to be "tighter," this is exactly what they mean. But don't cut it so far that you leave the operator no room to read — keep it comfortable.
On your mics, do you use gain to open up tone, or to manage sensitivity?