Provoke

Provoke is a keyword ability on creatures that allows a player to, during his or her attack step, untap a creature the defending player controls and require that creature to block the attacking creature with provoke. The block must still be legal, e.g. a non-flying creature cannot block a flying creature even when Provoke is used on it.

All tournament-legal cards with provoke were first printed in Legions.

From the Comprehensive Rules:

• 502.29. Provoke
  • 502.29a Provoke is a triggered ability. “Provoke” means “Whenever this creature attacks, you may choose to have target creature defending player controls block this creature this combat if able. If you do, untap that creature.”
  • 502.29b If a creature has multiple instances of provoke, each triggers separately.