The term 'chill-out' started to be popular in the mid 90's in the rave parties to name an area or room to rest from dancing and to relax. Chill-out areas are characterized by a warm light and atmosphere, and soft music styles like Ambient, Lounge, or Downtempo.
Chillout isn't dance music, it is listening music. For 40 years it has dedicated itself to the exploration of new audio textures and undiscovered sonic soundscapes, often inventing whole instruments and sound kaleidoscopes in its push for novelty. Because of this, the other genres have made a habit of consistently and constantly raiding the atmospheric mood and ambience of Chillout to employ in their own eclectic styles. But though it doesn't actively seek the spotlight as much as the others, Chillout will remain as the genre that was here first and will probably be here at the end, since it is the only one driven solely by an insatiable desire for innovation.
