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Pressure is one of two fundamental quantities describing fluid (hydraulic) processes. It corresponds closely to what we mean by it in everyday life. If we are submersed in a fluid, the fluid presses upon the surface of our body from all sides.
Pressure is equivalent to the potential of fluids (hydraulic potential), pressure differences are driving forces for different processes: by itself, a fluid flows from points where the pressure is high to points where it is lower.
Pressure is absolute (there is an absolute zero point of the hydraulic potential).
The isotropic part of the momentum current density tensor (mechanical stress tensor).

Liquid at rest in a gravitational field (hydrostatic pressure):
          p(h) = rho*g*h + p_a
rho: density of liquid, g: gravitational field, h: depth below surface, p_a: ambient pressure (air pressure at liquid surface).
Pressure of an ideal gas:
          p*V = n*R*T
V: volume of gas, n: amount of substance, R: universal gas constant, T: (absolute) temperature.
Energy current:
          I_E = p*I_V
I_E: energy current due to flow at pressure p, I_V: volume current at pressure p.


The pressure at the bottom of a 10 m column of water is about 1 bar; it is roughly the same as the pressure of the atmosphere at sea level.
Ambient pressure: Pressure of the environment, often the pressure of the surrounding air, or the pressure in (non-pressurized) parts of our body.
Pressure is often measured relative to ambient pressure.

  p, (P)
  [p] = Pa (Pascal), Pa = N/m^2; 1 bar = 10^5 Pa, 1 mmHg = 133 Pa
  hydraulic or fluid potential, hydraulic level, (isotropic part of) momentum current density tensor
Related to
  pressure difference, hydrostatic pressure, pressure force, chemical potential, water potential
(German) Druck; (French): pression