Primarily a scientific/technical term for the physical property of hardness, such as mineral hardness or material hardness. Also used figuratively for water hardness.
Diamond has extremely high hardness.
The water in this area has high hardness.
The water in this area has high hardness.
There is a tool for measuring the hardness of minerals.
硬度 is the technical, measurable term for hardness (e.g. Mohs hardness), while 硬質 is a broader descriptive term for something being hard or rigid.
硬度 (hardness) and 高度 (altitude, highness) are pronounced identically but refer to completely unrelated properties.
硬度 measures a material's hardness (resistance to scratching or indentation), while 強度 measures its overall strength or durability under stress.
Compound of 硬 (こう, 'hard') and 度 (ど, 'degree'), forming a technical term for the degree of hardness.