The pressure of a gas on a boundary surface equals to the force exerted by the gas on an area of a real surface divided by that area. For example a gas or a liquid in a container forces a pressure perpendiculary to the container walls; same proceeds from a firm body on its surface.
Nowadays the legal pressure units are Pascal as the SI unit, abbreviation Pa, and bar as a special unit designation.
p = F/A
1 bar=1000 mbar=10-5 N⁄m2 = 105 Pa.The unit commonly used in vacuum technology is the mbar.
In vacuum engineering the absolute pressure is standard, what means the pressure measured in relation to the absolute vacuum.
The ambient pressure p_amb is the atmospheric pressure on the Earth´s surface.
The normal pressure is according to DIN 1343 p_n = 1013,25 mbar.
The normal temperature ist T_n = 0°C = 273,15 Kelvin
Normal conditions refer to the established standard temperature T_n and standard pressure p_n of a solid, liquid or gaseous substance.
In technology and everyday life relative pressure is used, i.e. the pressure in relation to the current ambient pressure p_amb.