The meta is the set of strategies, characters, builds, and choices that the player community currently considers most effective in a game, shaped by the current version balance and by shared knowledge. Meta is short for metagame.
Meta is definitional before it is anything else: it names what is believed to work best right now, as a description of a community state, not as a law of the game. A character can be strong on paper and absent from the meta because players have not learned it, and a merely good strategy can dominate the meta because it counters the current popular picks. The meta is therefore a snapshot of knowledge and fashion layered on top of the actual game balance.
It changes for two reasons. Patches change the underlying numbers, with buffs and nerfs making old answers weaker and new options viable. Discovery changes knowledge without any patch at all, when a player demonstrates a technique or counter that spreads through videos, guides, and tournaments until the whole community adjusts. Tournament results matter disproportionately, because they show what survives under maximum pressure.
The word is used at every scale: the meta of one game mode, one rank bracket, or one region can differ from the global picture, since players face different opponents and copy different sources. Saying something is meta simply places it inside that current consensus of effectiveness. Saying it is off meta places it outside, with no judgement about whether the consensus is right.
In play"After the patch the meta shifted, and two characters nobody used are now in every match."