pascal (Pa) bar (bar) technical atmosphere (at) atmosphere (atm) torr (Torr) pound-force per square inch (psi) - still in widespread use in the US and Canada, notably for cars.
it is the "weight" that the air has, or the downward pressure it exerts on a surface, and can vary based on altitude above sea level, as well as temperature.It is measured in metric as kp or Kilo...