There are different definitions, non of them with exact reliable data for clinical use:
1, Histological: The “density” is calculated from the percentage of all bone tissue that is constituted by mineralized bone
2, Radiological: Mineral in bone is measured by x-ray translucency, defined as Hounsfield units.
3, Clinical: Tactile assessment of bone quality while doing bone preparation. In dental implantology classified from D1 (=dens) to D4 (porous).
These are 2 examples. For more, have a look in the Glossary!