If we’re to talk tech then it’s vital we all speak the same language, so here are some quick initial issues:

If there’s one word guitarists (and musicians in general) misuse, it’s gain. Gain, as you’ll find in any dictionary, means the ability of a circuit to increase the power or amplitude of a signal. Or as a parameter, we can define it as the (mean) ratio of an amplifier’s output signal to its input signal. So if we feed a 2V signal to an amplifier with a gain of 10 (which equates to 20dB), we would measure an output signal of 20V amplitude (provided there’s enough headroom, which is another topic).
For some strange reason, when guitarists say gain they really mean saturation! As in “A Dual Rectifier has more gain then a JCM800.”, which might be true, certainly is something we can determine but is not what the guy is thinking about when he says it. A Recto might sound more saturated then a classic Marshall but we’re really getting into subjective territory: a fully saturated sound is a square wave, and I’m sure people would agree that doesn’t sound as “gainy” as a cranked stack. So it’s not even the level of signal distortion but the specific harmonic content that results. A shrill sound that emphasizez the pick attack might sound more “gainy” then a dull “bluesy” sound, even if it comes from the same amp, at the same gain setting, being just a matter of EQ (or filtering, to be correct).
We might use alternate terms like overdrive or just drive instead of saturation, but never gain, for gain is as I said an intrinsic property of an amplifier stage while saturation is a consequence where more factors are involved. Par example, take a classic distortion circuit like the RAT: if you run it at 4V instead of 9V it will sound more saturated because the operational amplifier inside will have less headroom to swing the output signal, so it will “distort earlier”, i.e. with a lower input signal. Notice that this has nothing to do with the gain pot! The same principle (under-volting to decrease headroom) was apocriphally used by Van Halen on his Marshall, by employing a variac. But don’t get any ideas, ’cause there’s more than one way to skin a cat.

Another term that stabs my mind every time a musician uses it is warm sound. So far I haven’t been able to precisely determine what the hell this metaphor means, chiefly because everyone you ask will have his own vague explanation but most agree that it’s an elusive character, the “Je ne sais quoi” of one system or another. From a scientific standpoint it should be obvious why such descriptions are invalid. But if I must strive to make a determination, especially in the context of guitar tone, I’d wager it also has something to do with harmonic content. Warm is usually used in opposition  to harsh (shouldn’t it be cold?!), when trying to describe the sound of soft-clipping vs. hard clipping (more high-order harmonics), dominant crossover distortion and so on. Warm is thus inevitably associated with vacuum tubes, to the point that most would agree that anything with a tube in it sounds “warmer”, and the more tubes the better. Of course this is more nonsense since distortion (the particular dynamic and harmonic characteristic) again is determined by the whole circuit topology and operating conditions and not just by the particular active device used (as will be demonstrated in a later post).

I think that’s enough ranting about the modern musicians’ colorful vocabulary for now. If you have more terms you wish to clarify please mention them in the comments section. ‘Till next time, stay tuned and… in tune!