Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Pressure (symbol: P) is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure. Pressure is a tensor quantity, and has SI units of pascal; 1 Pa = 1 N/m2, and has EES units of psi; 1 psi = 1 lb/in2. Pressure is transmitted to solid boundaries or across arbitrary sections of fluid normal to these boundaries or sections at every point. It is a fundamental parameter in thermodynamics and it is conjugate to volume.