according to me limited knowledge of physics....

Wave Amplitude = volume
frequency = pitch
wave form = sound colour

and where does "gain" fits in to sound/EM wave? and why is it so impt to guitar sound?
tired of finger speed exercise and turned into slow blues improvisation lately
gain is amplification factor. if an op-amp has a gain of 1,000,000 (a realistic value), that means if you put in 1 volt, it tries to output 1,000,000 volts. obviously this isnt possible so the op-amp becomes saturated and clips. An op-amp cant output more than approximately the supply rails. some special types can output rail-to-rail voltages.

this is why we use negative feedback to limit gain. simple negative feeback is putting a resistor across the output and inverting input of the op-amp. the inverting input inverts the waveform so it cancels out with the output. the resistor is usually quite large, so all of the output never even makes it to the input. this is why it doesnt cancel out completely.

that's voltage gain. there's also dB gain, power gain, and others.
^ that's as good of an answer as you'll get. To relate the question back to you description of wave form - gain is a clipping of the peaks and troughs. Meaning instead of looking like a sine wave, it cuts off the parts of the wave with the highest amplitude and gives it a "square" shape.
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not ur fault but i am getting even more frustrated
so if my problem is that i want to keep the mid-output pickup in my guitars, but sometimes i need to boost the gain for more aggressive sound (as if the pickups are of highoutput), what kind of booster pedal do i need?? thx a lot
tired of finger speed exercise and turned into slow blues improvisation lately
If you have a tube amp that is already saturated (meaning the volume is high enough that it is naturally breaking up), then a boost pedal will help increase gain and volume by increasing the input load.

Otherwise you want to buy an overdrive or distortion pedal, which will give you a more aggressive sound and volume (ie for solos).
Suhr Custom, Flaxwood Rautia or Grosh Tele thru
HBE Medicine Bawl Wah
Analogman BiComp
Texas Two Step OD
Fulltone Ultimate Octave/Fuzz
Boss CE-2
TC Nova Delay
SLO-100, 65 London or Bogner Shiva
Quote by handlerb
^ that's as good of an answer as you'll get. To relate the question back to you description of wave form - gain is a clipping of the peaks and troughs. Meaning instead of looking like a sine wave, it cuts off the parts of the wave with the highest amplitude and gives it a "square" shape.

No, that's harmonic distortion.
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Gain usually indirectly leads to dostortion, as the tubes are trying to simply make it louder but can't. Gain in the propper sense is just increasing the signal stregth.