Decahedron/ˌdekəˈhiːdrən/ Definition 1 of 1Pro Tip 1/3Geometry Term A decahedron is often discussed in geometry classes and mathematical contexts. In our geometry lesson, we learned the properties of a decahedron. Pro Tip 2/33D Object Remember, a decahedron is a three-dimensional figure, not just a flat shape. The decahedron can be rotated to see all its ten faces. Pro Tip 3/3Regular vs. Irregular Decahedrons can be regular (all faces are the same) or irregular. She made an irregular decahedron with different polygonal faces.