Ambient music is meant to envelopp he who is listening but not so much as to take all of his attention away. Rather than auditioning the listener, it sets an atmosphere, as a musical wallpaper.
In other words, it is possible to enter its vast sound landscape and be taken on an intense journey through the senses.
As a musical term, Ambient music calls to long and harmonious arrangements built around instrumentations which melodies are intimately linked to a theme, sometimes so deeply that they become less obvious to differentiate. It's about an atmospheric context marked with etheral tones, which go along sound effects often picked in the environnement. Rhythm is not much pronounced, and often absent.
Almost all musical genres have compositions which fit the Ambient register, even sometimes to create a subgenre within. Its character is not based on rhythm (bass and percussions), vocals or rational instrumental chords.