The dancer’s grace, the flowing costume, and the ethereal melody created the impression that she floated through air.
characterized by an air-like insubstantiality
so delicate and insubstantial that it barely seems of this world — often beautiful
More rarely, but seen in older writings, ethereal can mean "heavenly or celestial" — as when Milton writes of an "ethereal messenger" in Paradise Lost. Similarly, it can mean "unworldly" or "spiritual".
In chemistry, there is a specialized meaning: "of or related to the chemical, ether"