Eighteen is a composite number with proper divisors 1, 2, 3, 6 and 9.

Also:

• A octadecagon has 18 sides
• A truncated tetrahedron and a great cubicuboctahedron have 18 edges
• A rhombicuboctahedron and a quasirhombicuboctahedron have 18 squares
• A small rhombihexahedron, a small dodecahemidodecahedron, a great rhombihexahedron, and a great dodecahemidodecahedron have 18 faces.