In the SI system of units, density is measured as kg/m³ (kilogram per cubic metre), but many people use the more convenient g/cm³ (gram per cubic centimetre[?]) or (equivalently) kg/l (kilograms per litre).
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 alternate 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 ton.
Perhaps the highest density known is reached in neutron star matter. The singularity at the centre of a black hole, according to general relativity, does not have any volume, so its density would be be seen as either infinite or nonexistent.
The densest naturally occurring substance on Earth is osmium, at about 22 kg/litre.
A table of densities of various substances:
Substance^{ }  Density (10^{3}kg/m^{3}) 

Gold  19.3 
Uranium  18.7 
Lead  11.3 
Copper  8.92 
Iron  7.86 
Aluminium  2.70 
Magnesium  1.75 
Water  1.00 
Air  0.0012 
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.
Relative Density Relative density, formerly called specific gravity, is a dimensionless quantity defined as the density of a substance divided by the density of water at standard temperature and pressure. By definition, then, the relative density (or RD) of water is 1, and the RD of osmium is about 22.
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.
Charge density is the electric charge per unit area or unit volume.
See also probability density function.
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