Denpa (電波), also denpa-kei (電波系) or denpa-san (電波さん), is a Japanese term for individuals who are disconnected or dissociated from the people around them. They may entertain wild fantasies and persecutory delusions or other strong beliefs, and their speech or actions may seem strange or incoherent to outside observers. Denpa literally means "electromagnetic wave", and the original sense of denpa-san was of someone who thought they were receiving voices, thoughts, or instructions directly to their mind via electromagnetic radiation. It is often used as a characterization in anime, manga, and Japanese light novels, such as "Ground Control to Psychoelectric Girl".
References
Denpa Wikipedia(Text) CC BY-SA