Gain
Definition
The amplification applied to an audio signal, typically at the input stage of a preamp, interface, or plugin. Gain is set before processing and determines how hot the signal enters the rest of the signal chain.
In Simple Terms
The volume knob at the very start of your signal chain — how loud the signal is before anything processes it. Set it right at the beginning and everything downstream works better. Set it wrong and nothing can fix it.
In Practice
Microphone gain is set on the preamp so the signal peaks around -18 dBFS, providing adequate level without clipping the input.
Common Confusion
Gain and volume are not the same. Gain controls the input level before processing; volume (fader) controls the output level after processing. Confusing the two leads to poor gain staging.