or SURFACE, in Geometry, the outside or exterior face of any body. This is considered as having the two dimensions of length and breadth only, but no thickness; and therefore it makes no part of the substance, or solid content, or matter, of the body. The terms, or bounds, or extremities, of a superficies, are lines; and superficies may be considered as generated by the motions of lines. Superficies are either rectilinear, curvilinear, plane, concave, or convex. A rectilinear superficies is that which is bounded by right lines; curvilinear superficies is bounded by curve lines; plane superficies is that which has no inequality in it, or risings, or sinkings, but lies evenly and straight throughout, so that a right line may wholly coincide with it in all parts and directions; convex superficies is that which is curved and rises outwards; concave superficies is curved and sinks inwards.