Invisibility was the condition of not being seen by other beings. It was a natural ability of certain beings such as the demon Vocah, though it could also be induced by certain spells.
Invisibility could also be "imposed", as was the case of Marcie Ross, who became invisible because she was perceived that way. It could be achieved through scientific means as well, as demonstrated by the Invisibility Ray created by The Trio. Certain spells could also induce invisibility.