Non-elemental damage https://irowiki.org/wiki/Non-elemental_damage

Non-elemental damage is a type of damage that is not affected by elemental modifiers.

• Examples of non-elemental damage include the physical melee auto-attacks of monsters and the physical melee auto-attacks of homunculi.

Most items and skills that are described as having the effect of reducing neutral damage actually reduce both neutral and non-elemental damage.

• For example, Raydric Card

However, ghost element armor like Ghostring Card does not reduce non-elemental damage.