(1) (Phys) A unit of pressure equal to 760 mm Hg at sea level. See: Air Pressure. (2) (Meteor) An atmosphere with a sealevel pressure of 1,013. 25 millibars, temperature of 15°C, and lapse rate of 6. 5°C up to 11 km (equal to 29. 92 in. Hg, 59°F, and 3. 5°F/1000 ft, respectively). : (3) (Med) Any gas mixture used for respiration