Just got to page 5 and got sick of Miros explanations that are false from my understanding, so let me explain what stereo means in a different way that other people have tried to say.
I'll take the example someone else used of marimbas and harps in an anechoic chamber.
Marimbas and harps and pianos and drums have multiple sources. There are multiple (I don't know the correct term for this on marimbas) keys that you can hit on a Marimba . If you were to stand directly in front of a marimba, you'd hear the notes move from left to right. Same with a piano, same with a harp, same with a drum SET. Because they are composed of multiple sound sources, but are considered one instrument, it's stereo.
Now. Put yourself in front of a singer or a guitar cabinet in an anechoic chamber. Move yourself forwards or backwards in the room, that sound will still sound like it's coming from one place. There is no left to right, up or down, ANY movement.
What YOU are saying is a sound becomes a stereo sound source because it's got reverb as a spatial cue. That's not true. If you were to record a vocal in an anechoic chamber as I've stated, with a stereo mic technique, unless the person is running from side to side in the room while talking, the sound will stay center. It will be mono when panned hard left and right. Do you see what everyone is saying yet? If you were to add reverb with a plug-in or reverb box, that sound will now be stereo (unless you're using mono FX of course).
If you record drums without moving the sources around (same as the vocal), you WILL get different stereo cues, because the snare isn't in the same spot as the hi hat or the ride. They will move left to right because of it, regardless of any processing you add to it.
In other words, a vocal and an acoustic guitar are both technically mono sources. Greg and others are not saying you can't record those in stereo for spatial cues because of things like reverb existing in environments, but those are still mono sources. One sound source. The only way you could REALLY call an acoustic stereo is if you're close enough to differentiate strings positioning by being that friggin' close to the guitar, but you get my point now, yes?
Some things are stereo sources because they have more than one element emitting sound. Multiple strings across 8 feet for a piano, a harp has multiple strings, marimba multiple "keys." That is what a stereo source is.
Thank you, I'll be here all week.
EDIT (as I'm reading the rest of the posts to give my views on them):
Farview's response on page 6 (#53) is essentially a more concise way of saying what I said, but I'm hoping by giving a longer explanation, it'll work to better the understanding.
That's how the internet works, right? xD
Jimmy's response on page 7 (#67) is also true, just a different way of stating what I'm saying. But in his example, things like keys and strings are the single mono source. But no one asks how to mic a single marimba key or a single piano string, they ask how to mic the instrument. Only kind of exception would be drums, where the overhead mics are stereo, but we often close mic the individual mono sources, but that's just to give them more presence in the recording if the stereo overheads didn't capture enough of any given element.
gecko zedd on page 8 (#74) kinda took it out of context when he said taking an instrument out into a field would still be stereo because birds are chirping and junk, when the guy's point was that an INSTRUMENT would be essentially "mono" at that point, and had nothing to do with outside influence. This is one of the silly semantics things that I see happening on here often, people aren't taking the spirit of the post into consideration, they are just literally interpreting every word meant as it's exact definition instead of trying to interpret what was said. Of course, then it was followed by people saying if you recorded in a literal vacuum there is no sound that will happen, when again, Beerhunter was just trying to make a point of if there are no spatial cues such as reverb or something, then a specific sound source could be mono.
Alright, all caught up on everyone's debating. Enjoy, I'm sure I'll be back to see how people try to tear apart my analysis.