Any class of polyhedron which is comprised of identical faces, usually a triangle*. There are surprisingly few polyhedra which fall into this category, five in fact, namely tetrahedron, octahedron, hexahedron (cube), icosahedron, and dodecahedron. These were described by Plato (hence the name), and were used to denote the five elements (earth, water, air, fire, and the heavens). BinarOne knows the mappings better than me.

Also, a solid which decides to abstain from sexual relationships.

*Okay, okay, there's a few more conditions involving edge/vertex transitivity which lead to other side-effects such as being convex, which ariels decided to point out, which I personally think are just in retrospect to make it so that Plato's five solids are the only platonic solids so that there's less catalogging to be done. Why doesn't ariels come and give us the full definition, hm?