The density of a fluid is a function of the pressure p,
temperature T and, for a liquid, the salinity 
,
defined as the proportion by mass of the dissolved salts [67].
Consider a particle at height z, where the density is 
,
in a fluid with no temperature or salinity
variation. The pressure at height z is given by the hydrostatic pressure
equation [67]
If the particle moves slightly to height 
then the pressure acting
on the particle and the fluid density will change to 
and
respectively and the particle will remain in equilibrium.
When we are considering internal waves we are concerned, not with the actual
density, but with the excess or potential density defined
[68, 69] as the density the fluid would have if compressed
adiabatically, with constant salinity, to a reference pressure 
.
This can be expressed [69]
where S is the entropy. This means that a fluid which has a density gradient produced solely by gravity has constant and no internal wave motion will occur. In practice internal waves only occur when there is a change in temperature or a change in salinity with depth.