succubus vs. incubus

noun

plural

succubi 
  1. a demon in female form, said to have sexual intercourse with men in their sleep.

  2. any demon or evil spirit.

  3. Archaic.,  a strumpet; prostitute.

noun

plural

incubi, incubuses 
  1. an imaginary demon or evil spirit supposed to descend upon sleeping persons, especially one fabled to have sexual intercourse with women during their sleep.

  2. a nightmare.

  3. something that weighs upon or oppresses one like a nightmare.