Top deck is a card-game slang term that typically refers to the card that has just been drawn from the top of the deck. The term can also be used as a verb; a player with zero cards in their hand is considered to be top-decking, and hoping to draw a card that will help their situation rather than using cards already in their hand.
A player who is top-decking is generally at a large disadvantage, especially if they aren't in a favorable board position. Certain cards such as , , , and especially are considered good general top-deck plays, though the best top-deck is typically a very situational card. For example, a mage with no minions that top-decked a against a full board of low-Health minions would be extremely lucky. However, if they were facing a single , they would likely be hoping to draw a , and may find a Flamestrike to be of minimal use.
Some decks are expected to be top-decking regularly, such as rogue decks, include more cards with card draw effects, that typically cost more mana for their effect, but provide additional card draw, such as . Since it typically takes a player several turns to empty their hand, such cards allow a top-decking player to avoid wasting mana while providing a way to cycle through their deck for more expensive, useful cards, as well as creating multiple effects in a turn. These cards are especially useful when top-decking in the late game, when the player has enough mana to play multiple cards each turn, but would otherwise lack the draw.
Top-decking is one of the strongest instances of RNG in Hearthstone, since a strong top-deck can often win the game, while a poor top-deck can lose it. As a result a victory which comes about through top-decking the right card at the right time is often ascribed to luck, despite the fact that it was only the course of the rest of the match, including the dozens of other instances of chance and myriad instances of skill and decision-making, that allowed the player to arrive at the situation in which their top-deck was able to win them the game.