Chill music is basically any kind of music that can be used as relaxation. Easy listening is an alternative word for chill-out music.
Chill-out has popular music characteristics in it. Though there is no straight line defining it as popular music but generally chill music is referring to relaxed and calm Pop music. You can find Classical, Jazz or New Age music inside a chill-out playlist, but the majority of the tracks are probably Pop.
Since chill-out has Pop music characteristics, it is very likely for it to be vocal. The main qualities of a chill-out can be mentioned as:
Clear and smooth melody.
Ambient sounding atmosphere.
The term chill-out is originated from the era “The White Room” in London. DJs “Jimmy Cauty” and “Alex Patterson, created some ambient mixes from their sources such as “Brian Eno”, “Pink Floyd”, “The Eagles”, “Mike Oldfield”, and “War”. the purpose of these ambient mixed was to give dancers a chance to “chill out” from the fast-tempo music played on the main dance floor.
In the early 2000s, DJs in Ibiza's Café Del Mar began creating ambient house mixes that drew on jazz, classical, Hispanic, and New Age sources. The popularity of chill-out subsequently expanded to dedicated satellite radio channels, outdoor festivals, and thousands of compilation albums. "Chill-out" was also removed from its ambient origins and became its own distinct genre.