Pressure is the ratio between the magnitude of force applied to a surface and the size of the surface area it is being applied to.
The instrument used to measure pressure is called a pressure gauge. There are several types of pressure gauges, such as the barometer (used to measure atmospheric pressure), vacuum gauge (used to measure low pressure).
When referring to pressure, we most commonly mean blood or atmospheric pressure.
The base unit of pressure is the pascal, a unit equal to one newton per square metre. In some places, units such as pound per square inch and bar are also used.