The bar is a metric (but not SI) unit of pressure exactly equal to 100 kPa, or 100,000 Pa. It is about equal to the atmospheric pressure on Earth at sea level.
The bar and the milibar were introduced by the Norwegian meteorologist Vilhelm Bjerknes in 1909, defining the bar as one megadyne per square centimeter.
Units derived from the bar are the megabar (symbol: Mbar), kilobar (symbol: kbar), decibar (symbol: dbar), centibar (symbol: cbar), and millibar (symbol: mbar or mb). These are not SI or cgs units, but they are accepted by the BIPM for use with the SI. The bar is legally recognized in countries of the European Union.
The bar unit is considered deprecated by some entities. While the BIPM includes it under the class "Non-SI units accepted for use with the SI", the US National Institute of Standards and Technology (NIST) includes it in the list of units to avoid, except for "limited user in meteorology", and recommends the use of kilopascals (kPa) and megapascals (MPa) instead. The International Astronomical Union (IAU) also lists it under "Non-SI units and symbols whose continued use is deprecated." Bar(g) is a unit of gauge pressure, i.e., pressure in bars above ambient or atmospheric pressure.