Hardness of a material is rated using Mohs Scale of Mineral Hardness. Each mineral is measured against the scale by finding the hardest material that the given material can scratch, or the softest material that can scratch the given material.
Mohs Scale of Mineral Hardness
A real life example of applying this scale would include your fingernail, which has a hardness of 2, which can easily scratch soap, which has a hardness of 1. The copper coin in your pocket is about a 3, where your pocket knife is probably a 5. Your bedroom window, however fragile it is, has a hardness of ~5.5 (note that the scale does not account for brittleness).