Destroy
Destroy is a game action that means sending a card from the battlefield to the graveyard. It can only be done by following a game rule or card effect.
A card must be on the battlefield to be destroyed (see Discard for moving cards from the hand to the graveyard).
When a face-up card that was summoned (i.e. did not Fizzle) is destroyed, before it is sent to the graveyard, its "When destroyed" effects are activated, if any.
Destroy is part of the Card Lifecycle. Destroying a card you control is referred to as a Sacrifice.