1Existing or occurring beneath the stars or (in ancient and medieval cosmology) below the sphere of the fixed stars; (hence also) terrestrial, earthly.
2In various forms of mysticism: designating or relating to the lower part of the astral plane or realm.
3= substellar. rare.
Origin
Mid 18th century; earliest use found in William Warburton (1698–1779), bishop of Gloucester and religious controversialist. From sub- + astral.