A solid is that which has length, breadth, and depth.

A face of a solid is a surface.

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
- prisms. XI.Def.13 and proposition XI.39
- pyramids. XI.Def.12, XII.3 and the following propositions
- cones and cylinders. XI.Def.18 through XI.Def.24, XII.10 and the following propositions
- spheres. XI.Def.14 through XI.Def.17, and propositions XII.17 and XII.18
- octahedra. XI.Def.26, XIII.14
- cubes. XI.Def.25, XIII.15
- icosahedra. XI.Def.27, XIII.16
- dodecahedra. XI.Def.28, XIII.17