Affinity

Affinity is a keyword ability that reduces the mana cost of a spell by the number of permanents of a certain type that player controls. It appeared in the Mirrodin block.

Description
The ability is always written as "Affinity for [something]". The most common instance is "Affinity for artifacts" (see Frogmite). However, a cycle of Golems (consisting of Dross Golem, Oxidda Golem, Razor Golem, Spire Golem, and Tangle Golem) appeared in the Darksteel expansion, each with affinity for a particular basic land type. Hum of the Radix functions as an "anti-affinity" enchantment, increasing by 1 the cost of all artifact spells for each artifact its controller owns. Mycosynth Golem grants affinity to artifacts for all artifact creature spells cast by its controller, in addition to having affinity for artifacts itself.

Almost all tournament-legal cards with affinity were printed during the Mirrodin block, and contributed heavily to the Second Combo Winter. The mechanic was considered for a return in the Scars of Mirrodin block, but rejected because it had been so destructive for the metagame in its first appearance.

Affinity appeared as a one-off in War of the Spark with Tezzeret, Master of the Bridge</c>.

Affinity was reintroduced in Modern Horizons 2, using new variants such as "Affinity for tokens" (Junk Winder</c>).

Unkeyworded affinity
With cost-reduction being so powerful and the format-warping effects of affinity for artifacts, the mechanic has not been revisited. However, single cards have been printed that fit the rules text that have not been keyworded to save keyword learning space. , and  are examples of those that still use artifacts.

Other objects include:
 * Gate Colossus</c> (Gates)
 * Embercleave</c> (attacking creatures)
 * The Circle of Loyalty</c> (Knights)
 * Brine Giant</c> (enchantments)
 * Hamza, Guardian of Arashin</c> (creatures with +1/+1 counters, also grants it to your other creature spells)
 * Icebreaker Kraken</c> (snow lands)

Ten spells that care about Party have "Affinity for creatures in your Party", but the wording for Party and the wording of Affinity mix poorly. In a similar vein:
 * Stratadon</c> has "Affinity for Domain"; Draco</c> and Scion of Draco</c> have "Affinity for Domain" twice.
 * Valiant Changeling</c> has "Affinity for creature types".
 * Drag to the Underworld</c> and Marshmist Titan</c> have "Affinity for devotion to black"; Daybreak Chimera</c> has "Affinity for devotion to white"
 * Khalni Hydra</c> has a unique version where the cost reduced is rather than, but otherwise has "Affinity for green creatures".

Rulings

 * The affinity ability reduces only the amount of generic mana you pay. It can't reduce how much nongeneric mana you pay.
 * Affinity can't reduce the cost to cast a spell to less than.
 * Affinity doesn't change the spell's mana cost or converted mana cost. It just changes how much mana you pay to cast the spell.
 * The cost reduction is set before you have to pay any costs for the spell. If you sacrifice an artifact while paying the spell's costs, that artifact still counts toward the cost reduction.

Notable cards with affinity
<c>Frogmite</c>, <c>Myr Enforcer</c>, <c>Thoughtcast</c> - used in affinity decks