I always assumed pressure was the collective weight of what's above you, so water pressure is the weight of all the water directly above you, plus 1 atmosphere of air pressure.

If it's particle density, well that explains why pressure increases with temperature, and it explains atmpospheric pressure because gas is compressible.

If pressure has no effect on the density of a liquid, then how does particle density increase? That's why I'm confused, because I agree with what you said renton and that doesn't tally with the idea that pressure is simply a measure of particle density.