Lethal

Lethal is a term used in Hearthstone to refer to damage sufficient to defeat the enemy hero and win the game. "Having lethal" is used as a measure of imminent and unavoidable success, similar to "check-mate" in chess.

Discussion
A player who "has lethal" is capable of dealing sufficient damage to the enemy hero to destroy them, ending the game. In theory, any player who has lethal will win that turn. However, players often predict that they "will have lethal" on the next turn, with the possibility for actually dealing fatal damage hinging upon the opponent's actions. This is similar to "check" in chess.

For example, a warlock may have a Flame Imp on the board, plus a Doomguard and a Drain Life in their hand, in theory allowing them to deal 10 damage next turn. If the opponent's Health is at or below 10, and they have no Taunts in play, the player may therefore consider that they have lethal. However, if during their turn the opposing player heals their hero, places Taunts on the board, or otherwise obstructs the warlock's plans, the chance to deal fatal damage will have been lost.

Having lethal is an especially important concept for aggro deck and burst-oriented decks. These decks may hold back on dealing damage until the opportunity arises to deal lethal damage in a single turn. This can surprise the opponent, whose relatively high Health may lead them to consider themselves on solid ground; if the player does not anticipate this type of play, they may focus on opportunities to attack rather than to reinforce their defences, unaware of potential damage the enemy is holding back. "Miracle" rogues are a good example of a deck type which deals minimal damage through most of the game, aiming to deal lethal damage in a single burst of 20 or more damage. In these cases, it can be very effective to withhold playing cards and dealing damage until lethal is available; playing out damage piece-meal can allow the opponent time to respond and counter the attack.

Missing lethal
In theory, any player who has lethal on their own turn should win the game, short of unexpected Secrets. However, in practice players sometimes miss opportunities or overlook card combinations that could have allowed them to win the game that turn. This is often known as "missing lethal".

While most misplays in Hearthstone are difficult to prove, due to random events and incomplete information, a missed lethal is often an easily provable misplay. The range of effects and possible plays can make calculating lethal in Hearthstone tricky, especially given the limited time available each turn, and even professional players frequently miss lethal. Players focusing too much on their own defence or countering the opponent's plays may overlook the possibility of winning that turn by instead focusing exclusively on hero damage.

As a result, it is a good idea to start later turns by calculating the total direct damage available on board and in hand, especially when the opponent is low on Health. Some games are decided by a single turn, with the defeated party having had lethal the next turn; this can be used as the highest measure of a game's closeness.