Damage

From WoWWiki

Damage, or DMG, is a reduction in health or an action that reduces health from an attack. For example, a weapon attack or spell attack that reduces the health of the target is damage, but losing a health boosting buff (i.e., stamina increasing spells, like Blood Pact) is not considered damage.

Losing mana from an attack can also be considered damage, but generally is not referred to in that way since reducing mana to 0 (zero) will not cause death; nor do Drain Mana-only spells interrupt casting or other actions prone to damage, such as bandaging.