Density (symbol: ρ - Greek: rho) (ISO 31: volumic mass) is a measure of mass per unit of volume. The higher an object's density, the higher its mass per volume. The average density of an object equals its total mass divided by its total volume. A denser object (such as iron) will have less volume than an equal mass of some less dense substance (such as water).
where ρ equals density, m equals total mass, and V equals volume.
Formerly mass and volume were linked by defining the gram to be the mass of one cubic centimeter of water at 4°C which meant that water had density 1 kg/litre. However, using one cubic centimeter of water as a standard for one gram is problematic due to the possibility of mass loss from evaporation as well as changes in density with temperature. For this reason alternative definitions of the meter and kilogram have been developed, which can be reproduced more reliably in a laboratory. Because of slight changes in the metre and kilogram due to these new definitions, the density of water at 4°C is not quite exactly 1, but 0.99995 kg/litre. A cubic meter of water thus weighs approximately one metric tonne.
Note the low density of aluminium compared to most other metals. For this reason, aircraft were made of aluminium in the past. Also note that air has a nonzero, albeit small, density. Aerogel is the world's lightest solid.
Density may denote how much of a certain substance, object or occurrence is present per unit area or volume.
Often used is population density, meaning how many people per square kilometre (or square mile) on average live in an area.
The density of discrete entities such as people is difficult to characterise as a continuous quantity.
Geographers and mathematicians have made a number of attempts to formalize the concept of population density.