Pressure is the force per unit area exerted by the weight of the atmosphere.
Source: Ferguson, Sue A. 2001. Smoke Management Meteorology. A chapter in Smoke Management Guide. by Achtemeier, Gary, Brenner, James D., Core, John E., Ferguson, Sue A., Hardy, Colin C., Hermann, Sharon M., Jackson, Bill, Lahm, Peter, Leenhouts, Bill, Leuschen, Tom, Mutch, Robert E., Ottmar, Roger D., Peterson, Janice L., Reinhardt, Timothy R., Seamon, Paula, and Wade, Dale. A Publication of the National Wildfire Coordinating Group.