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Euclid's Elements, Book XI, Definitions 1 and 2

Definition 1

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
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