hard|ness «HAHRD nihs», noun.
1. the quality or condition of being hard.
2. the quality of water holding mineral salts in solution that make it unfit for washing purposes.
3. the comparative capacity of one mineral to scratch, or be scratched by, another mineral. The degree of hardness of a mineral is expressed with reference to the Mohs scale, ranging from 1 (talc) to 10 (diamond).
4. the ability of a missile site or missile to withstand enemy aerial attack.
Useful english dictionary. 2012.