- mass per unit volume, measured in kg/m3
- determines buoyancy: objects float when less dense than the surrounding fluid
- water density at 4 C: 1000 kg/m3, the standard reference
- varies with temperature and pressure; gases are highly compressible
- air at sea level: ~1.225 kg/m3
- Earth average: ~5515 kg/m3, indicating a dense metallic core
- relative density (specific gravity) compares a substance to water
- used in material selection, fluid mechanics, geophysics, and astrophysics
- population density and information density extend the concept beyond physics