Pressure
Pressure is defined as force per unit area. In Systeme Internationale (SI) units, the fundamental unit of pressure is the Pascal (Pa; kg m-1 s-2), defined as 1 Newton (kg m s-2) per square meter. Pressure may also be considered a unit of mechanical energy density, where 1 Pa equals 1 Joule per cubic meter. Other units of pressure include dyne cm-2 (= 0.1 Pa); bar (= 105 Pa); atmosphere (= 1.0133 x 105 Pa); and millimeters of mercury ( = 1.333 x 102 Pa).
Pressure may be imposed on a surface of any phase (gas, liquid or solid) by another body of any phase. Bodies of any phase may also be characterized by an internal pressure. Atmospheric pressure at sea level is equal to 101.3 kPa.
"Negative Pressure" describes a condition in which a body exists under a state of tension, such as stretched rubber, or water under tension in trees.