Defending Player

Defending Player is the non-Active Player that is being attacked by the opponents creature.

Details
The "defending player" only exists during the attack step.

When an ability refers to the "opponent" during an ability when you attack (usually an Attack Trigger), it refers to the defending player.

In a multiplayer format, only the defending player can block with their creatures. In a 1 on 1 game, it non-active player is always the defending player.