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. The ability is always written as "Affinity for" something. The most common instance is "Affinity for Artifacts" (see Frogmite), however a cycle of Golems has one artifact creature with affinity for each basic land type.

All tournament-legal cards with affinity were printed during Mirrodin Block.

From the Comprehensive Rules:

• 502.31. Affinity
  • 502.31a Affinity is a static ability that functions while the spell is on the stack. “Affinity for [text]” means “This spell costs you {1} less to play for each [text] you control.”
  • 502.31b The affinity ability reduces only generic mana costs; it doesn’t reduce how much colored mana you have to pay for a spell. Affinity can’t reduce the cost to play a spell to less than 0.
  • 502.31c If a spell has multiple instances of affinity, each of them applies.