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. 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. 
All tournament-legal cards with affinity were printed during the Mirrodin block.
- The affinity ability reduces only the amount of generic mana you pay. It can't reduce how much colored 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.