1. A type of stress characterized by uniformity in all directions. As a measurable on a surface, the net force per unit area normal to that surface exerted by molecules rebounding from it. In dynamics, it is that part of the stress tensor that is independent of viscosity and depends only upon the molecular motion appropriate to the local temperature and density. It is the negative of the mean of the three normal stresses. The concept of pressure as employed in thermodynamics is based upon an equilibrium system, where tangential forces vanish and normal forces are equal. 2. In meteorology, commonly used for atmospheric pressure. 3. In mechanics, same as stress. 4. See radiation pressure.