Definition of pervert noun from the Oxford Advanced Learner's Dictionary

BrE BrE//ˈpɜːvɜːt//
; NAmE NAmE//ˈpɜːrvɜːrt//
(informal perv) a person whose sexual behaviour is not thought to be normal or acceptable by most people synonym deviant a sexual pervert See related entries: Immoral
Word Originlate Middle English (as a verb): from Old French pervertir, from Latin pervertere, from per- ‘thoroughly, to ill effect’ + vertere ‘to turn’. The current noun sense dates from the late 19th cent.
See the Oxford Advanced American Dictionary entry: pervert