Peter Pan syndrome is a pop-psychology term used to describe an adult who is socially immature. The term has been used informally by both laypeople and some psychology professionals in popular psychology since the 1983 publication of The Peter Pan Syndrome: Men Who Have Never Grown Up, by Dr. Dan Kiley. (Kiley also wrote a companion book, The Wendy Dilemma, published in 1984.)
Peter Pan syndrome is not listed in the Diagnostic and Statistical Manual of Mental Disorders, and is not recognized by the American Psychiatric Association as a mental disorder.