The bar (symbol: bar) is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the kilobar (symbol: kbar), decibar (symbol: dbar), centibar (symbol: cbar), and millibar (symbol: mbar or mb). They are not SI units, nor are they cgs units, but they are accepted for use with the SI. The bar is widely used in descriptions of pressure because it is only about 1% smaller than "standard" atmospheric pressure, and is legally recognized in countries of the European Union.
Except for the power of ten, the definition of bar fits in the sequence of SI pressure units (Pa, kPa, MPa), namely, 1 bar ≡ 100,000 Pa = 100 kPa = 0.1 MPa. This is in contrast to the well-known unit of pressure, atmosphere, which now is defined to be 1.01325 bar exactly. As a rule of thumb, a bar is almost equal to an atmosphere.
The bar and the millibar were introduced by the British meteorologist William Napier Shaw in 1909. William Napier Shaw was the director of the Meteorological Office in London from 1907 to 1920.
Barg is a unit of gauge pressure, i.e. pressure in bars above ambient or atmospheric pressure; see Bar (unit)#Absolute pressure and gauge pressure.