Degree of hardness or firmness of something; physical stiffness, and figuratively firmness or honesty.
このパンの硬さはちょうどいい。
The firmness of this bread is just right.
木材の硬さを測る装置を使った。
We used a device to measure the hardness of the wood.
彼の態度には仕事に対する硬さが感じられる。
One can sense a certain rigidity/seriousness in his attitude toward work.