Offering

Offering is a keyword ability that allows a player to sacrifice an appropriate permanent to play a card with Offering as an instant and at reduced mana cost. It was introduced in Betrayers of Kamigawa in the five card "Patron" cycle, and has not been reprinted since that set. The cards are:

Patron of the Akki, Patron of the Kitsune, Patron of the Moon, Patron of the Nezumi and Patron of the Orochi.

From the Comprehensive Rules:

• 502.42. Offering
  • 502.42a Offering is a static ability of a card that functions in any zone from which the card can be played. “[Text] offering” means “You may play this card any time you could play an instant by sacrificing a [text] permanent. If you do, the total cost to play this card is reduced by the sacrificed permanent’s mana cost.”
  • 502.42b The permanent is sacrificed at the same time the spell is announced (see rule 409.1a). The total cost of the spell is reduced by the sacrificed permanent’s mana cost (see rule 409.1f).
  • 502.42c Generic mana in the sacrificed permanent’s mana cost reduces generic mana in the total cost to play the card with offering. Colored mana in the sacrificed permanent’s mana cost reduces mana of the same color in the total cost to play the card with offering. Colored mana in the sacrificed permanent’s mana cost that doesn’t match colored mana in the colored mana cost of the card with offering, or is in excess of the card’s colored mana cost, reduces that much generic mana in the total cost.

For your reference [CR 409.1a,f]:

• 409.1a The player announces that he or she is playing the spell or activated ability. It moves from the zone it’s in to the stack and remains there until it’s countered or resolves. In the case of spells, the physical card goes onto the stack. In the case of activated abilities, the ability goes onto the stack without any card associated with it. Each spell has all the characteristics of the card associated with it. Each activated ability on the stack has the text of the ability that created it, and no other characteristics. The controller of a spell is the player who played the spell. The controller of an activated ability is the player who played the ability.
• 409.1f The player determines the total cost of the spell or ability. Usually this is just the mana cost (for spells) or activation cost (for abilities). Some cards list additional or alternative costs in their text, and some effects may increase or reduce the cost to pay. Costs may include paying mana, tapping permanents, sacrificing permanents, discarding cards, and so on. The total cost is the mana cost, activation cost, or alternative cost, plus all cost increases and minus all cost reductions. Once the total cost is determined, it becomes “locked in.” If effects would change the total cost after this time, they have no effect.