To adopt a strategy or tweak one's current strategy in a game based on the perceived strategy of one's opponents or fellow competitors. Also known simply as "metagaming," although that term generally does not connote the same level of a shift in strategy as the full term does. See metagame.
It is most commonly used in collectible card games, in which there are tournaments in which one designs and plays one's own deck. In these tournaments, a large portion of one's strategy (the contents of the deck) will be determined before playing in the tournament at all, so it is important to be prepared. Players have even been known to scout the pre-tournament crowd before finalizing their deck in order to get an idea of what types of decks other people will be playing, as well as any uncommon card choices ("tech") that might not have previously been considered.
An example of this that led to a very large reward was a Magic: The Gathering deck named "The Solution" developed and played by Team Gozilla at Pro Tour: Tokyo in 2001. After analyzing all the cards available in the format, Invasion Block Constructed, the team decided that most people would have a red component to their deck in order to remove the opposition's creatures. In order to combat this, their maindeck contained many creatures that could not be removed this way, as well as one that could protect other creatures from them. Normally such narrow cards would be relegated to the sideboard, if included at all, to swap into the deck for games 2 and 3 of the match. The team members ended up doing very well against decks that featured red removal, and 6 of the 7 members made the cut for the second day, compared to only about 1/3 of the original field. The most important achievement of the deck, however, was its ability to also do well against decks whose designers chose to not include the strong red cards most players did, and thus were unaffected by Gozilla's playing of the metagame. One of the team members, Zvi Mowshowitz, won the event and $30,000, winning in the finals against a deck that sported no red cards.
Playing the metagame works when it is completely unexpected, and able to hold its own against decks that are not the intended target of the shift in strategy. If it's expected, it's merely another part of the metagame that others will be considering. If it doesn't work well against decks not susceptible to the narrow cards necessary to combat the major deck type(s), it won't be able to last through a 10+ round tournament. Those looking to play the metagame successfully must tune their deck to the point where it seems to have the desired results against the entire field of decks, while keeping their developments secret before the big tournament.
This term is also encountered in games where players are only supposed to bring themselves to the table, but might end up bringing knowledge of and relationships with other players as well. In these cases, playing the metagame is often extremely frowned upon. Serious players of the game will expect the game to be played in a vaccuum, and not have one's strategies based on the events of previous games with the same players. This is most often encountered in Diplomacy, many of whose enthusiasts are very serious about having fair games, given the deterministic nature of the game. Players already familiar with each other have what is considered an unfair edge in negotiations over players who have never met their potential allies and enemies before.
Pro tour: tokyo 2001 coverage available at http://www.wizards.com/sideboard/event.asp?event=PTTOK01