suc·cu·bus 
(sŭkyə-bəs) also suc·cu·ba (-bə)
(sŭkyə-bəs) also suc·cu·ba (-bə)n. pl. suc·cu·bus·es or suc·cu·bi (-bī′, -bē′) also suc·cu·bae (-bē′, -bī′)
1. A female demon supposed to descend upon and have sexual intercourse with a man while he sleeps.
2. An evil spirit; a demon.
[Middle English, from Medieval Latin, alteration (influenced by Late Latin incubus, incubus) of Latin succuba, paramour, from succubāre, to lie under : sub-, sub- + cubāre, to lie down.]