the world > space > relative position > condition of being external > covering > a layer > [noun] > an overlying or top layer (3)
A stratum or layer situated over or on something; an overlying or superficial stratum. Also figurative and in figurative contexts.
That which is put on the top of anything to complete it; a top layer; cf. top, v.1 9.
= superstratum, n. 1; spec. an overlying or outer layer of transparent rigid material, as on the sunward side of a photovoltaic cell.