Ninjutsu

Ninjutsu is a keyword ability which was introduced in Betrayers of Kamigawa, and represents the sneakiness of the Ninjas, by allowing a creature that is attacking and unblocked to be exchanged with a Ninja in the controlling player's hand, such that the Ninja is attacking and unblocked instead of the original creature, which is returned to the controller's hand.

The keyword har only appeared in Betrayers of Kamigawa, and exclusively on black or blue cards.

From the Comprehensive Rules:

• 502.43. Ninjutsu
  • 502.43a Ninjutsu is an activated ability that functions only while the card with ninjutsu is in a player’s hand. "Ninjutsu [cost]" means "[Cost], Reveal this card from your hand, Return an unblocked creature you control to its owner’s hand: Put this card into play from your hand tapped and attacking."
  • 502.43b The card with ninjutsu remains revealed from the time the ability is announced until the ability leaves the stack.
  • 502.43c A ninjutsu ability may be played only while a creature in play is unblocked (see rule 309.2f). The creature with ninjutsu is put into play unblocked. It will be attacking the same player as the creature that was returned to its owner’s hand.

For your reference [CR 309.2f]:

• 309.2f An attacking creature with one or more creatures declared as blockers for it becomes a blocked creature; one with no blockers becomes an unblocked creature. The creature’s status remains unchanged until the creature is removed from combat or the combat phase ends, whichever comes first. (Some effects can change a creature’s status.)