Kicker

Kicker is a keyword ability that allows the player to pay an optional cost when playing a spell to achieve an additional effect.

All tournament-legal cards with kicker were printed during Invasion block.

From the Comprehensive Rules

• 502.21. Kicker
  • 502.21a Kicker is a static ability that functions while the spell is on the stack. “Kicker [cost]” means “You may pay an additional [cost] as you play this spell.” The phrase “Kicker [cost 1] and/or [cost 2]” means the same thing as “Kicker [cost 1], kicker [cost 2].” Paying a spell’s kicker cost(s) follows the rules for paying additional costs in rules 409.1b and 409.1f–h.
  • 502.21b Objects with kicker have additional abilities that specify what happens if the kicker cost is paid. Objects with more than one kicker cost have abilities that correspond to each kicker cost.
  • 502.21c If the text that depends on a kicker cost being paid targets one or more permanents and/or players, the spell’s controller chooses those targets only if he or she declared the intention to pay the appropriate kicker cost. Otherwise, the targets aren’t chosen at all.
  • 502.21d A card with kicker may contain the phrases “if the [A] kicker cost was paid” and “if the [B] kicker cost was paid,” where A and B are the first and second kicker costs listed on the card, respectively. This text just refers to one kicker cost or the other, regardless of what the spell’s controller actually spent when paying the cost. In other words, read “if the [A] kicker cost was paid” as “if the first kicker cost listed was paid,” and read “if the [B] kicker cost was paid” as “if the second kicker cost listed was paid.”

For your reference [CR 409.1b,f-h]

• 409.1b If the spell or ability is modal (uses the phrase “Choose one —” or “[specified player] chooses one —”), the player announces the mode choice. If the player wishes to splice any cards onto the spell, he or she reveals those cards in his or her hand. If the spell or ability has a variable mana cost (indicated by {X}) or some other variable cost, the player announces the value of that variable at this time. If the spell or ability has alternative, additional, or other special costs (such as buyback, kicker, or convoke costs), the player announces his or her intentions to pay any or all of those costs (see rule 409.1f). You can’t apply two alternative methods of playing or two alternative costs to a single spell or ability. Previously made choices (such as choosing to play a spell with flashback from his or her graveyard or choosing to play a creature with morph face down) may restrict the player’s options when making these choices.
• 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.
• 409.1g If the total cost includes a mana payment, the player then has a chance to play mana abilities (see rule 411, “Playing Mana Abilities”). Mana abilities must be played before costs are paid.
• 409.1h The player pays the total cost in any order. Partial payments are not allowed.
  • Example
    You play Death Bomb, which costs {3}{B} and has an additional cost of sacrificing a creature. You sacrifice Thunderscape Familiar, whose effect makes your black spells cost {1} less to play. Because a spell’s total cost is “locked in” before payments are actually made, you pay {2}{B}, not {3}{B}, even though you’re sacrificing the Familiar.