Denpa song (電波ソング) is a Japanese internet slang word for a J-pop song with weird and incoherent lyrics or a deliberate out-of-tone singing. The lyrics is so nonsensical that even native Japanese listeners can hardly grasp the point. The origin of the slang denpa (電波), literally means "radiowave", comes from a random killer case in 1980s in which the tweaker (methamphetamine user) murderer claimed that he "received a radiowave command and that's why he slaughtered random people". During 1990s some Japanese musicians and novelists used denpa to express the paranoid state of mind, and this slang spread through their works.
Any J-pop song with weird lyrics can be counted as a denpa song, but usually a song from the anime or vocaloid genre is called denpa song. Ika and CosMo@Bousou-P are some of the famous denpa-style creators in the Vocaloid music scene.