Tempo

Tempo is an unofficial term that describes which way the "momentum" of a match is going, as well as a label for decks which aim to keep tempo on their side. It is one of the major types of advantage a player may have over the opponent. Tempo is concerned with the player's control over the game in the immediate and short-term, particularly with minions played on the board, whereas card advantage is about having more options and resources than the opponent over the long run, including cards in the hand. Given enough time, card advantage naturally turns into tempo advantage, since maintaining momentum requires having cards that can respond to enemy obstacles.

Definition
While tempo is often considered to be a common term and fairly simple to understand, it is also often considered to be among the more difficult things in Hearthstone to describe by definition rather than by examples or through a practice run.

Many suggestions have been made for verbally describing this use of tempo, some of which are below:
 * The momentum of the match. Probably the original definition and among many unofficially accepted "right" ones. However, its imprecision can lead to misunderstandings and odd interpretations causing confusion in discussion.
 * The change of the board state in a given time (usually either a play or a turn, as in "a good/bad tempo play/turn"). What this definition has in its favor is its simplicity, but this very simplicity means that it does not cover all the uses of this term.
 * The speed at which a player is reaching their win condition. An alternate interpretation of the original definition that is especially useful when discussing decks that don't usually win by gaining board control or that don't aim at reaching that goal at the start of the game.
 * The value of the effect on the board state divided by the mana used. The main upside of this definition is its being in the form of a mathematical formula, because in the future it could be applied to calculations.

Evaluation
Tempo is strongly associated with board control. It could for example be defined as the change of board state in a given time. A player with a clear tempo advantage will have multiple minions in play which the opponent is not able to easily remove. On the other hand, if both players end up losing and replacing all their minions each turn, the tempo is close to even.

Often the advantage can be judged by whether a player could kill the opponent's minion(s) without sacrificing all of their own minions. For instance, if a paladin has a Spider Tank and a Shielded Minibot at the end of their third turn while the opponent has no minions, the player would likely be able to kill the opponent's 3-drop, such as another Spider Tank, while keeping at least one of their two minions alive. Since the paladin enters each turn with living minions and plays even more minions during the turn, the pressure is on the opponent to respond to those minions with their own cards, without having the benefit of minions already in play. Thus, the tempo is on the paladin's side.

It is possible to control the flow of the game even without a statistical advantage of minions in play, if the opponent's actions are sufficiently restricted, such as with effects like Taunt and freeze. Because the opponent is not free to take desired actions (such as using a minion to kill important enemies, or dealing damage to the player), while the player has versatile options (from making advantageous trades to simply racking up damage against the opponent), the player can still be said to be winning the tempo game.

Gaining and reclaiming tempo
The limited supply of mana is the primary restriction on playing cards in the beginning of the game, so making good use of all of it is the primary factor in generating tempo early. This includes playing drops each turn that efficiently consume all available mana, trading up and using removal when necessary to make one's own mana "do more work" than the opponent's, and sometimes using effects that decrease costs or increase the actual amount of mana available.

Various mechanics have different effects with regard to tempo:
 * Weapons are an excellent tool for controlling the game, since their multiple charges allow them to remove multiple enemy cards from the board using only one similarly-priced card.
 * "Fast" minions that have an immediate effect (such as a Battlecry, Charge, or Rush) are generally better for tempo than standard "slow" minions that must wait a turn to act (which gives the opponent the opportunity to respond to them efficiently).
 * For example, Fire Elemental can be used to kill a 3-health minion instantly, or Kor'kron Elite may charge into an important 1/3 minion. In addition to these immediate effects, both cards leave minions on the board which can be further used to gain control.
 * Taunt is sometimes considered a "fast" ability because it immediately restricts the attack options of enemy minions, often forcing them into unfavorable trades. However, because the effect is not truly immediate, the opponent still has the opportunity to neutralize the minion with some other effect.
 * Battlecries, passive effects, or spells that help other minions trade efficiently can also be effective tempo boosters. However, these typically require that a player already have some minions in play, and may be difficult to use if significantly behind.
 * A Shattered Sun Cleric turning a 2/3 ally into a 3/4 could allow it to kill an opposing 2/3 in one hit, leaving two 3/2 minions for the player and nothing for the opponent - a solid tempo advantage on turn 3.
 * A Raid Leader's extra attack power could enable a whole field of minions to trade up for more valuable ones, solidifying a tempo advantage or breaking a stalemate.
 * The healing from an Earthen Ring Farseer could allow a damaged Chillwind Yeti to remain in play after hitting an opposing Bloodfen Raptor, foiling an opponent's plan to kill it at little cost.
 * Mana acceleration can be a huge contributor to tempo by increasing the amount of mana available compared to the opponent. Wild Growth trades 3 mana (and its associated tempo) early for a gradually increasing tempo advantage in subsequent turns due to the additional Mana Crystal. The Coin, Innervate and Preparation provide one-time tempo boosts by increasing the number or power level of cards playable in a single turn. Of course, mana acceleration is only as valuable as the use that the mana is put to. If the resulting cards are used inefficiently or the mana is simply wasted, then it has not contributed to the tempo.
 * Just as acceleration increases tempo, mana penalties can significantly hurt it. Most common is the Overload ability, which trades a slightly stronger effect (and tempo gain) on one turn for reduced mana (and tempo loss) on the next turn. Arcane Golem gives the opponent additional mana and therefore tempo while Felguard slows the player's momentum in multiple subsequent turns. Players must judge whether the increased power of these cards is worth the tempo drawback.
 * 0-cost spells such as Backstab are clear increases in tempo: the caster may do everything else that was planned for the turn, plus a bonus effect for no cost at all. However, these cards usually have a fairly small effect and are therefore poor for card advantage.
 * Area of effect spells can be effective at "catching up" on tempo once behind. However, it should be noted that expensive AoE spells often prevent the caster from playing minions on the same turn. This can allow the opponent to take back the tempo by simply casting more minions on their own turn.
 * Single target removal often only breaks even on momentum. It can only be played defensively (it cannot be played before the opponent has played a target for it), only removes one piece of the opponent's power, and tends to cost almost as much as the target removed. However, when used thoughtfully it can enable advantageous trades which build up tempo, or prevent the opponent from making such trades.
 * Especially cheap removal may directly cause a tempo gain if the cost of the removal is far lower than the minion removed. For instance, Naturalize is a 1-for-3 exchange in card advantage but is so cheap that its tempo gain may help a druid win anyway.
 * Return effects such as Sap are a cheap alternative to removal. While disadvantageous in terms of cards (since the enemy card is not actually destroyed), the low cost makes them much easier to play in the same turn as other cards.
 * The standard Hero Powers are the worst tempo abilities available, since their effects are far smaller than cards that cost the same amount. This is the tradeoff for their high card advantage (they consume no cards at all). As with removal, however, if used thoughtfully in the right situation, damage-dealing hero powers may at least prevent an opponent from gaining too much tempo by trading a one-health minion into a much stronger one.

Player health and damage
Having the tempo on one's side often leads to damage against the opponent, since any minions not used for trading can be used to attack the face. It is possible for a player with a significant tempo advantage in the early game to kill the opponent before any card disadvantage can be gained or put into full effect. Or, they may eventually lose the tempo, but not before dealing enough damage that the opponent can be killed with a sudden burst later on.

As the latter scenario illustrates, player health is not a reliable direct indicator of tempo. A player at 30 health facing a player with 15 health might have the tempo, but they could just as easily have done the damage in the first few turns of the game, then lost their advantage after the opponent made a series of favorable trades or efficient removals.