Demon

"'How do you know a demon is lying? His lips are moving.'"In the Georgina Kincaid series, a demon is an angel that has broken one of the laws of Heaven and thus 'fallen' from grace.

In the grand scheme of things, neither angel or demon is permitted to directly interfere with the lives of mortals. Either side breaking this law frees up the opposing side to undo whatever unlawful interference took place (something that can be seen in action in Succubus Revealed).

Like Angels, all Demons are powerful, however there is a varying degree of power from demon to demon, likewise, abilities and strengths seem to similarly vary. The strongest demon that rules over and oversees the 'running' of a territory is called an Arch-Demon and each Arch-Demon is selected by the 'administration' in Hell to be paired with the territory best suited to them.

Two of the known ways an angel can fall and become a demon are love/sex with a human (Jerome) and by killing a fellow angel (Yasmine).