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.