A Keplerian disk is a disk of material that follows Kepler's laws of motion in conjunction with a central body, which could be a circumstellar disk, or rings around a planet, both of which generally follow the laws for the most part.
The Keplerian disk concept is a somewhat ideal (a first approximation), as forces other than the gravity of the central body can make portions of the disk diverge from normal Keplerian orbit. Example forces are radiation pressure from the central star or pressure from heat in the disk material. Such non-gravitational pressure helps keep the material from falling into the star, thus once things have found a stable state, the material is moving slower than it would in a pure Keplerian orbit. Gravitation between portions of the disk is also likely to be discounted.