Density is the ratio between the mass and the volume of an object.
In equilibrium, if two substances don't mix, the substance with lower density will float on the one with higher density.
Among substances with the lowest density are helium and aerographite, while objects such as black holes and neutron stars have some of the highest densities.
Density varies depending on temperature and pressure. With solid matter, the variation is generally low, but it can be substantial with gasses.
Water reaches the highest density at 4°C.
The base units for density are the kilogram per cubic meter and the ounce per cubic inch.