The meta
Also said as Metagame, Tier list, Flavour of the month
What most people are playing at the moment, and the choices that makes best in reply. It is the game above the game: you don't only pick what's strong, you pick what beats what everyone else is picking.
It changes with no patch at all, as players copy what wins and then start beating it. A healthy one keeps moving. A stale one means everyone has stopped thinking and started copying a list.
Say it
“Don't patch yet. The top deck has been on top for only a week, and its counter is already rising. Check the shares again in ten days.”
Where you’ve seen it
Hearthstone players follow deck tier lists built from ladder data, such as the Vicious Syndicate reports, and choose their decks by what they expect to meet.
Not to be confused with dominant strategy
A dominant strategy is best no matter what. The meta only has to be best right now, against what's common, and it can be overturned by something that was bad last week.