Tempo

Tempo is a term to describe the pace at which one plays threats. Some people would consider tempo as one of the fundamental components of Magic.

Concept
The concept of tempo is to dissect the more intricate means of how and when lands and spells are played, as opposed to the "race" which approaches aggressive strategies haphazardly. A player who has a strong understanding of tempo can often put the game state in their favor without having to resort to copious amounts of card advantage or card utility for contingency's sake during the game. Tempo is not specific to any one color, nor does it favor a specific color for having the "best tempo", as tempo exists merely to scale the game's pace and allow its reader a better understanding of how to seek victory.

The term has its roots in musical notation, where it means the rate of movement between sounds in relation to one another. It was later adopted into Chess where it found its origin in Magic: The Gathering as the gaining or losing of position and productivity in relation to victory. Often when in relation to a player's deck, the tempo is simplified to "Speed" which refers to the absolute "fastest" tempo a single deck can produce (not necessarily the one most optimal for the match.)

Bringing the definition of tempo back into Magic terminology, one should imagine the hypothetical where each player has a sequence of draws where each player makes the first eight land drops and casts a spell with maximum mana each turn - say, one creature a turn from turns one to seven. If each creature trades with the opposing creature at each juncture, no damage is done and the tempo is even; at the end of the starting player's eighth turn, they would have no cards in hand. Taking this out of these ideal assumptions, at any point in which a player misses a play, the aggressor cannot attack, or the defender cannot block because their creatures do not line up, that player falls behind in tempo.

Alternatively, casting two spells in a turn becomes another potential source of tempo; on turn 4, the attacker uses a two-mana combat trick to win the three-mana creature combat and plays another two-mana card. While a four mana creature may be superior to the two mana creature, the aggressor retains an active creature and adds another one. This is where "gaining a mana advantage" comes from. This can only go so far; if the defender's four-mana play dominates both the aggressor's creatures, the aggressor's four-mana play needs to immediately answer it (with a removal spell), or else some tempo is lost.

Winning with tempo
Tempo is often used in conjunction with mana efficiency. Aggro decks are built around maximizing their own tempo by "going under" other decks. They cast more spells and use their mana more efficiently in the early turns and, by doing so, try to kill the opponent before the opponent's more powerful game plan comes online. Midrange decks can also do this, but this is reliant on the metagame. Control decks, of course, only care about tempo to the extent that it allows them to survive long enough to where card advantage becomes more important - rarely do control decks ever put in tempo spells as they suffer in the late game as answers. One of the aspects of control decks and tempo is how the play pattern of board wipes develop - while doing nothing for five turns then wiping the board may work, the damage in the meantime may become sufficient that attempting to spot-remove each threat is not good enough.

Tempo decks
"Tempo decks" operate on the premise that any type of disruption spell can be converted into damage, given a superior board state. While all aggressive decks do this to a degree, tempo decks lean heavier on the conditional disruption that is more punishing to particular answers. For example, while Lightning Bolt is an excellent answer to Centaur Courser, it is not against Baloth Gorger (unlike Unsummon) or Day of Judgment (unlike Spell Pierce). The price paid in this is that in situations when the best answer is a standard removal spell, the tempo deck will fall behind on the exchange, sometimes lethally.

For this reason, tempo decks are relatively rare in high-level competitive play as their gameplan is heavily reliant on drawing not only consistently but also in the right order - the threats are typically weaker and less resilient than an aggro deck, and their tempo spells also tend to suffer if behind on board. The most successful tempo decks were ones where the main disruption were high-quality counterspells - Cryptic Command in Lowryn-Alara Faeries, Mana Leak in Scars-Innistrad Delver, and Siren Stormtamer in Ravnica Allegiance Mono-Blue. Delver of Secrets</c> stands out as one of the most prevalent tempo cards in the game, though many Legacy Delver decks no longer build like tempo decks.