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Having thought carefully, I really can't imagine any scenario in practical audio operations where it would matter if someone confused gain and level. If anyone can think of such a situation, then I'd love to know. But it doesn't hurt to have these things clear in one's mind, so here is a simple statement that should help...
Gain=change in level
So a signal has a certain level, whether it be sound pressure, voltage or digits. If you do something that changes the level, you have applied gain. So if the level of a signal is -26 dBFS and you apply 6 dB of gain, the signal level rises to -20 dBFS.
The word 'gain' of course implies more of something. Like 'profit' means more money. The opposite of 'profit' is 'loss'. The opposite of 'gain' is 'attenuation'. So if we want to make a signal lower in level, then we have to apply an attenuation. If the level of a signal is -6 dBFS and you apply 12 dB of attenuation, the signal level drops to -18 dBFS. With the magic of negative numbers, which have been with us for more than 2000 years now, we can indeed talk about negative gain just as easily as attenuation. So once again if the level of a signal is -6 dBFS and you apply -12 dB of gain, the signal level drops to -18 dBFS.
In electronic audio, there is a way of thinking that positive gain is provided by active devices - devices that take electricity from a power source and use it to boost the signal. Level in a downwards direction - attenuation - can be controlled by passive devices that need no power source - a couple of resistors will do nicely.
So an electronic engineer may think in terms of controlling gain with active devices and controlling level with a passive device such as a fader.
But how can a fader provide +10 dB of gain at the top of its scale? Simple - by actively applying that 10 dB of gain before the fader. When the fader is set to 0 dB, it attenuates the already-boosted signal by 10 dB.
I do feel a mild sense of irritation when, for example, I see a 'gain reduction' meter on a compressor. It should be 'level reduction' or 'attenuation'. Or it could just be labeled 'gain' and calibrated in negative values of decibels.
At the end of the day however it isn't that much to worry about in practical audio operations. Just keep in mind that 'gain=change in level' and you'll be fine!