Pressure is a measure of force per unit area and is typically expressed in units such as pounds per square inch (psi), pascals (Pa), or bar. In the context of fluid mechanics, pressure is the result of the force exerted by a fluid (liquid or gas) on the walls of its container or any object immersed in the fluid.

There are several types of pressure:

1. *Absolute Pressure:* Absolute pressure is the total pressure exerted by a fluid, including atmospheric pressure. It is measured relative to a perfect vacuum (zero pressure).

2. *Gauge Pressure:* Gauge pressure is the pressure measured relative to atmospheric pressure. It is the pressure difference between the absolute pressure and the atmospheric pressure.

3. *Differential Pressure:* Differential pressure is the difference in pressure between two points in a fluid. It is often used to measure flow rates, levels, and filter conditions in fluid systems.

Pressure can be measured using various instruments, including manometers, pressure gauges, and pressure transducers. These instruments convert the pressure into a readable form, such as a dial reading or an electrical signal, that can be used for monitoring and control purposes in various applications, including industrial processes, automotive systems, and environmental monitoring.