1. Mass per unit volume. Commonly measured in g/cm3 or kg/m3, often without the units being expressed explicitly. Bulk rock densities vary mainly because of porosity and are generally in the range 1.9–2.8 g/cm3. The ratio of density to that of water is called specific gravity and is dimensionless. 2. Frequency of occurrence. 3. The equivalent position of a color on a gray scale. See Figure C-7a. 4. A measure of the degree of blackening of an exposed photograph after development.