Millibars (mbar) and pounds per square inch (psi) are two units that measure pressure. They can be converted at a rate of 1 psi per 68.95 mbar, or 1 mbar per .0145 psi.
Neither the mbar, a metric unit, or the psi, an Imperial unit, are used by the International System of Units (SI). Rather, the pascal (Pa), a derived unit defined as Newtons per meter squared, is the standard. One mbar is equivalent to 100 Pa.
The name "bar" comes from the Greek word "baros," meaning "weight." One bar is approximately equal to atmospheric pressure at sea level, so meteorologists use the mbar to describe changes above or below this standard.