n. [ Litho- + sphere. ] (Phys. Geog.) (a) The solid earth as distinguished from its fluid envelopes, the hydrosphere and atmosphere. (b) The outer part of the solid earth, the portion undergoing change through the gradual transfer of material by volcanic eruption, the circulation of underground water, and the process of erosion and deposition. It is, therefore, regarded as a third mobile envelope comparable with the hydrosphere and atmosphere. [ Webster 1913 Suppl. ]