A sub-genre of electronica. Unlike most electronica genres, ambient is not intended to be danced to; it often has no explicit downbeat, and when it does, the tempo is usually far too slow for dancing. Ambient specializes in drones, slowly evolving textures, and gradually shifting harmonies and timbres. Often, there are only a few chord changes in a five-minute ambient piece, or perhaps none at all. It focuses largely on the timbral characteristics of sounds, often organized or performed to evoke an "atmospheric", "visual" or "unobtrusive" quality.
Unlike most musical genres of any sort, ambient can trace both its name and its origin to a specific work, that being Brian Eno's "Ambient 1: Music for Airports", released in 1977. At the time, Eno was recovering from having been seriously injured in an automobile accident, and he stated that the music fit both his weakened physical condition and his mental state at the time. Eno released a series of "Ambient" albums (not all consisting of electronic music), before abandoning the genre in the early '80s.
At about that time, a group of artists working in a (non-electronic) style known as New Age picked up the ball. New Age combined meditative moods, somewhat similar to Eno's work, with quasi-religious aspects. Some artists who had worked with synths in the late '70s, such as Eddie Jobson and Patrick O'Hearn, combined the New Age mood with electronics and the '70s electronic "space music" performed by bands such as Tangerine Dream, and developed the immediate precursor to modern ambient. This didn't have its own name at the time, but was simply lumped in with the New Age genre. (A side branch off of this was the genre now known as planetarium music.)
Ambient was briefly put aside at the dawn of electronica in the late '80s, as dance-oriented music ruled the scene. However, some clubs began installing "chill rooms" where burned-out dancers could go to sit down and get away from the beat for a while, and the DJs working those rooms (Alex Paterson, to name one) began mixing the '70s and '80s ambient music. Current artists noticed and began composing new music true to the genre, but with the modern sensibility, and ambient was re-born. Today, ambient retains its place, somewhat off to the side of the electronica category, as it is often physically off to the side in the club chill rooms. A number of artists who compose and mix dance-oriented styles also work in ambient, usually under other names.
Ambient music, unlike other forms of "background music", is intended to enhance acoustic and atmospheric idiosyncrasies in the sound environment. Whereas conventional background music is produced by stripping away all sense of doubt and uncertainty (and thus all genuine interest) from the music, Ambient Music retains these qualities. And whereas their intention is to "brighten" the environment by adding stimulus to it (thus supposedly alleviating the tedium of routine tasks and leveling out the natural ups and downs of the body rhythms) Ambient Music is intended to induce calm and a space to think.
Ambient Music must be able to accommodate many levels of listening attention without enforcing one in particular; it must be as ignorable as it is interesting. As a genre it originated in the United Kingdom at a time when new sound-making devices such as the synthesizer, were being introduced to a wider market. Brian Eno was an early pioneer of ambient music. The Orb and Aphex Twin gained commercial success with ambient tracks. Ambient compositions are often quite lengthy, much longer than more popular, commercial forms of music. Some pieces can reach a half an hour or more in length.