Definition: A millibar (abbreviated as mb) is a non-SI unit used for measuring air pressure. The term comes from bar, which was the original unit used for measuring pressure. (Bar is derived from the Greek "baros," which means "weight.")

While the millibar isn't recognized as the official unit of pressure, for reasons of convenience, it's frequently used by U.S. meteorologists and weather reporters.

(FYI the official, SI unit of pressure is the Pascal.)

The millibar is easily converted into other units of pressure by using the below relationships:

• 1 mb = 1/1000 of a bar
• 1 mb = 1 hectopascal = 100 Pascals