I’d say that “meta” deck is generally a deck that meets at least one and typically both of two basic criteria to some degree:
- High play rate. Typically it needs to be high enough that many opponents will recognize it based on the cards played, even if there are some tech choices/switches that have been made.
- High enough power/winrate to be a consideration that any other player trying to choose/build/refine a competitive deck needs to consider it.
The play rate consideration is probably the main feature. Something like Tickatus Control Warlock would probably be considered a meta deck solely through its play rate, since its win rate has actually been really bad except against a couple classes like Priest (so it sort of fits the power criteria as well, but only from that one particular matchup).
And sometimes even a deck that has a lower play rate can be strong enough or polarizing enough in its matchups that everyone needs to keep it in mind, even if it only occasionally appears. But even then it still needs to be a widely enough recognized option that other players would care about it.