Otk dating definition
In Hearthstone, can means cards that affect the Hand (like Dirty Rat), Deck (like Gnomeferatu) or even Weapons (like the Oozes).These are the elements of Hearthstone that cannot be easily interacted with, they require specific cards to affect them, unlike the Battlefield and it's minions, where any minion/weapon attack can kill them, any spell can directly or indirectly remove them.
The Advanced Format banlist is updated every six months to reduce the viability of OTKs. Since Duelists usually start with 4000 Life Points in the anime, an OTK is declared when all 4000 are depleted in one turn.
OTK, One Turn Kill Deck: A Deck whose list includes a combination of cards that when played from hand in a single turn can deal damage equal to or higher than the total starting health of the opponent, or that destroys the opponent immediately with a card effect.
(Think of the Four Horsemen from Paladins, or Exodia in Yu-Gi-Oh) It is a deck which can, in every single game, achieve this condition.
In Hearthstone, any deck that can deal 30 Damage or destroys the enemy hero from an empty battlefield at the start of your turn, which can achieve this every single game regardless of the order in which the deck was drawn and as long as the player survives, is an OTK deck.
OTK decks can only be stopped by cards designed specifically to interact with them.