Meta-gaming means different things to different people, and more importantly, the line where it becomes odious or a problem is specific to the individual. Snowy's definition is pretty much spot-oin, though. In essence, meta-gaming is the idea of viewing the game from outside the game, the 'meta-game', which is where the player resides, not the character. Everyone, by definition, does this. It is quite simply impossible not to...the question is where the line is drawn for such behavior.
For example: if my players attack a monster, and hit it when they hit an AC of 35 but miss it when they hit an AC of 37, they're going to use metagame knowledge to reason out the AC of the creature. Certain characters then may decide not to attack, as they know that short of a natural 20, they won't successfully hit, potentially wasting the action (when they could aid to attack or something else).
On the one hand, much metagame behavior is pure gamist theory, similar to playing monopoly or chess and knowing the options available to you. On the other hand, the D&D system doesn't model certain details that strongly, and metagame knowledge may be a reflection of ingame knowledge. If I rephrase the above situation as the party watching as their most competent warrior finds himself hard-pressed to get past the creatures swift reflexes and natural armor-plating, it isn't wrong to believe that the rogue will try something other than a head-on assualt....especially considering that adventurers have knowledge and skills that the player does not.
Amethal's example is another good one: the character certainly would know he could substitute spells, even though the player didn't. It's up to the individual DM how to handle that situation, but I wouldn't penalize one player for helping another player with a new system, myself. D&D is a big game...no one remembers all the rules all the time, unless maybe you're Hypersmurf.
Another example: a character sees a combat in progress, and a monster takes its AoO on a character. Player 2 says "
Oh, he's taken his AoO for the turn, so I run past him and attack him from behind!" Is this metagaming? Yes. Is it inappropriate? That's up to the individual group. In my game, it's not invalid, as I view this to be the purview of professional adventurers.
Of course, my group takes nothing for granted, and hasn't since I warned them (several years ago, now) that monsters and enemies could be considerably different than the base MM creature. Does that Ogre have Combat Reflexes? Only one way to find out. A Painful Way, at that.
What sort of behavior is the player engaging in that you find questionable?