A solid is that which has length, breadth, and depth.
Definition 2
A face of a solid is a surface.
Guide
The 28 definitions at the beginning of Book XI serve Books XII and XIII as well.
The first two definitions correspond to definitions I.Def.2 and I.Def.3 for a line and its ends, and definitions I.Def.5 and I.Def.6 for a surface and its edges.
Some examples of solids that appear in Books XI through XII are:
parallelepipedal solids. See proposition XI.24 and the following propositions