Metagaming is breaking the rules of a game. It's called cheating in other games.
When you look at the cards of another player in Poker, that's metagaming.
EDIT: In D&D the most common form of metagaming is listening in on information the DM is parceling out to another player whose character is in a position to learn it when yours is not.
I think we can agree to disagree on the first point.Nope. In D&D the most common form of metagaming is to have read the monster manual and to recognise the challenges the DM is throwing at you. The second most common form of metagaming is to be genre-savvy, Scream-style and know the checklist of what not to do in a horror movie.
And there's a reason that any DM I've played with (and I as a DM) leave the room with the player for that sort of info rather than making them compartmentalise.
I think we can agree to disagree on the first point.
As to the reading of the MM, DMG, any adventure being run, etc., I agree. I was totally thinking of stuff happening at the table. I mean, we would need some scientific studies and lots of honest polling data to really get an idea of "which is the most", but gut check? Yeah, I think anything that adds up to peeking behind the screen is more common.
But that is unequivocally breaking the rules of the game. At least the one's we use.
As to the other stuff, games don't really belong to genres. What you're talking about is act of playing a game. Only using expectations from other games to lead you.
Metagaming is breaking the rules of a game. It's called cheating in other games.
When you look at the cards of another player in Poker, that's metagaming.
When you read the answers to the Jeopardy questions before being a contestant for Mr. Trebek, that's metagaming.
When as a participating athlete you don't pole vault in the Olympic Games, but tell the judges you did, that's metagaming.
Breaking rules is not bad in and of itself, but it changes the game difficulty.
EDIT: In D&D the most common form of metagaming is listening in on information the DM is parceling out to another player whose character is in a position to learn it when yours is not.
Actually, Metagaming as defined in early D&D is part of player cheating. The new definition is different. As early D&D has nothing to do with expressing a personality or telling a story, that definition doesn't apply.This is just false and combined with awful examples.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.