An effect (効果 kōka) is an occurrence in the game caused by a spell, ARTS, PIECE, or ability. An effect may cause cards to move between zones, change a card's state, power, or text, change the rules of the game, and so on.
The abilities of certain cards may allow them to be unaffected by effects. This protects only against a specifically defined subset of effects; see the Unaffected page for details. In addition, some effects may counter the effects of a spell or PIECE, which causes those effects to do nothing.