Either one of the two types of spirits known as succubi or incubi. A succubus is a female spirit while incubus is male; both are believed to be able to have sex with people of either gender. It is also said they can be summoned through occult means.
Definition 2: Someone really really good at having sex or someone obsessed with sex; or both.
She gave up on human guys and started fucking with a sex demon.
She is always so horny I swear she's a sex demon.