To me, "metagaming" is using out of game concepts in-game. The oft-cited example of the PC confronted with a pit trap who says "There has to be a way around this because the DM has to give us a way across" is metagaming. What I don't get is the people (my own group, included) who would NOT consider it metagaming if the same player said "There has to be a way around this because whoever designed it would have to have a way across." The same exact thing in different words, but one is "roleplaying" and one is "cheating". The fact of the matter is that we're still playing a GAME. Using the terms of said game should not be considered cheating of any sort. That's like saying someone who says "squares" instead of "streets" (or roads, or whatever) in Monopoly is "metagaming".
Example: At 5th level, my group confronted an evil cleric who summoned some skeletons. I was playing a duskblade with the background of being trained in various combat maneuvers (i.e. Power Attack/Combat Expertise/Combat Reflexes), and I was confronted with a skeleton. I used power attack on it, when another player rudely questioned my reason for using it. I thought he was talking to me OOC (technically he was, he asked Wayne, not my character) so I said "That way I do enough damage to overcome its damage reduction" at which point I was promptly yelled at by the rest of the group for metagaming. When I got mad and questioned (citing the fact that at 5TH LEVEL I would reasonably know what the weakness of a CR1 skeleton was), I was told that if I had said "Because it's a skeleton, thus brittle, so a powerful strike should break it", they wouldn't have considered it metagaming.
I do NOT consider it metagaming for someone else OOC to remind me OOC about something my character would know. As fusangite has pointed out, I play only several hours as my character; whereas in the context of the D&D world he plays himself 24-7. I may forget that the assassins guild I hate so much is called the Crimson Hawks, but my character certainly does not; if Bob reminds me out of game, there shouldn't be a problem. Now if Bob OOC knew that the guy I was befriending was a spy for the Crimson Hawks, but I did not, THEN it's metagaming (unless Bob's PC is near mine and can tell me).
Personally, I find metagaming to be an excuse to penalize players for not analyzing every little thing from the mind of their character. People also take it too far.. by personal experience I have had DMs who would ask for Knowledge (Nature) checks to know what common monsters like Hobgoblins and Bugbears are (at level 6, mind you), and if you did not have it as a class skill he would basically tell you "It's a large, hairy goblinoid" and you would get jumped for metagaming if you said "It's a bugbear" (actually, someone would ask you rudely "How does your CHARACTER know that?"). In that same example, we were told of a Wyvern ("A dragon, but it has two legs and attacks with its tail"); in a case like that I could see requiring Knowledge since to the common person, a winged reptilian thing is a dragon, no matter what. However, if you want to get technical as to what "metagaming" is, asking someone "How much HP do you have left?" is metagaming. So is "What's your AC?" because these are abstract roleplaying game terms, not terms used in the context of the fictional world we are playing in.
In short, I hate the term metagaming and see no point in it. It never was an issue when I played 2nd edition, so I have a hard time not doing it in 3rd, even though it results in a lot of arguments with my group.
Regards,
Wayne