1. thermodynamically, the normal force exerted by a homogeneous liquid or gas, per unit of area, on the wall of the container.

2. force exerted per unit area. Note: The terms head and pressure often are used interchangeably; however, head is the height of a fluid column supported by fluid flow, while pressure is the normal force per unit area. For liquids, it is convenient to measure the head in terms of the flowing fluid. With a gas or air, however, it is customary to measure pressure on a column of liquid.