If your term isn't defined here, ask about it on alt.guitar.amps !
The term BRIGHT generally refers to a very high frequency boost. It's usually implemented as a switch that passes extra highs at some point, such as past the volume or tone controls.
-partly from Tom Lanik
Take any song from an old AC/DC album. That *IS* "crunch".No further definition is needed, because that is the definition. Distortion (as it is commonly referred to) and crunch rarely cross paths. Crunch distortion is created by a hot signal slamming into a non-master volume high wattage amp at high volumes. Think humbucker into Marshall 100 watter, no effects, no preamp distortion, etc...
To add a little more detail to my original post - this is a sound the sits right on the edge of distortion. You can get a nearly clean tone by simply picking lightly, or a big, ballsy distortion sound by hitting a power chord. I do think you need humbuckers to get there, though I'm sure someone will disagree.
-Ross Stites (from two posts)
-partly from Tom Lanik
GAIN is the amount of amplification in an amplifier (technically measured as a voltage change or in dB as a ratio). Too much gain is known as overdrive (distortion). Too little gain either means not enough volume, or noise as you add more gain stages.
Gain is often used interchanably with volume, which is incorrect usage.
-mostly from Tom Lanik
(This is not only PMG's fault, it's his definition. -ed)
VOLUME is the actual output level from a preamp, channel or amp.
Volume is often used interchanably with gain, which is incorrect usage.
-mostly from Tom Lanik
NOS stands for New Old Stock. It refers to something that has never been used, so it's in new condition, but has just been sitting around in someone's parts inventory (stock). May be good, may not. Be careful when dealing with anyone of unknown reputation; some people will claim used parts are NOS. And there is a lot of NOS garbage (reject tubes from long ago that should have been destroyed, or NOS caps that have dried out) on the market.
The best online terminology guide for amps I have found is courtesy of Randall Aiken at http://www.aikenamps.com/ (click on "Tech Info", then "Introductory", then "A glossary of common amplifier terms").