1. The compactness of a substance; the ratio of mass to unit volume, usually expressed as g/cm3 (kg/m3 in the SI). 2. The quantity of electricity on a given surface or in a given time per unit of volume. 3. Radiologic physics the opacity to light of an exposed radiographic or photographic film; the darker the film, the greater will be the measured density. 4. Clinical radiology a less exposed area on a film, corresponding to a region of greater x-ray attenuation (radiopacity) in the subject; the more light transmitted by the film, the greater the density of the subject will be; this is not actually the opposite of sense 3, because one concerns film density and the other subject density.