Fluid pressure occurs in one of two situations:
- an open condition, such as the ocean, a swimming pool, or the atmosphere; or
- a closed condition, such as a water line or a gas line.
Pressure in open conditions usually can be approximated as the pressure in "static" or non-moving conditions (even in the ocean where there are waves and currents), because the motions create only negligible changes in the pressure. Such conditions conform with principles of fluid statics. The pressure at any given point of a non-moving (static) fluid is called the hydrostatic pressure.
Closed bodies of fluid are either "static," when the fluid is not moving, or "dynamic," when the fluid can move as in either a pipe or by compressing and air gap in a closed container. The pressure in closed conditions conforms with the principles of fluid dynamics.