Destroy is the game action of putting a card from the field or magic stone area to its owner's graveyard.
A card on the field can only be 'destroyed' because of the following reason:
- A result of spells or ability that has the word 'destroy' on it.
- Lethal damage (Getting damage, either by battle or effect, so that the damage is equal or more than its DEF).
- State-based effect (Such as having 0 DEF because of ATK/DEF reduction effect).
Number 1 is applied to each card type, while number 2 and 3 are applied to Resonator or J-Ruler.
For J-Ruler case, there's an exception. If something would destroy J-Ruler cards in Ruler area (Astral Ruler), it does nothing. However, if it's a Ruler in a Ruler area, it'll move into its owner's graveyard if the effect says "destroy ruler' specifically. Such as Time of Ragnarok.
If an effect says a card cannot be destroyed, all effects that try to destroy it are ignored, and it doesn't fulfill the condition to be destroyed by rule process.
If a card is sent from the field into a graveyard for any reason other than described above, such as Banish. It isn't considered as being destroyed.