Delve

Delve is a cost-reduction keyword ability. It was introduced in Future Sight.

From the Comprehensive Rules

• 502.64. Delve
  • 502.64a Delve is a static ability that functions while the spell that has it is on the stack. “Delve” means “As an additional cost to play this spell, you may remove any number of cards in your graveyard from the game. Each card removed this way reduces the cost to play this spell by {1}.” Using the delve ability follows the rules for paying additional costs in rules 409.1b and 409.1f–h.
  • 502.64b Multiple instances of delve on the same spell are redundant.

• Delve can't reduce the colored portion of a spell's cost.

• When using delve, a player can remove a number of cards in his or her graveyard that exceeds the generic mana cost of the spell. Doing so will reduce the generic mana cost of the spell to 0, though the colored mana cost of the spell will still have to be paid.

• While playing a spell with delve, you can sacrifice a permanent as part of a mana ability, then remove that card from your graveyard as part of delve. So a Blood Pet, for example, can be used twice to help play a delve spell. The same is true if the spell has an additional cost such as "sacrifice a permanent" or "discard a card."