The phrase radial drift clearly applies to orbits shifting inward or outward in relation to the host, but is commonly used in discussions of planet formation for a particular kind of inward drift of solid objects within a protoplanetary disk. The gas orbits at lower than Keplerian speed due to gas and radiation pressure, and objects sufficiently small to be fully coupled, orbit with it. But somewhat larger objects are not carried along, and only orbit at Keplerian speed, a bit faster, and the relative motion with the gas results in a headwind. Being slowed, the object drifts inward. Another source of such drift is dynamical friction, i.e., from Lindblad torque. Radial drift is the basis for the radial-drift barrier and is an element in streaming instability.