For 10 points, name this fluid variable, one of the primitive variables of atmospherics along with temperature, fluid velocity, and density.
One non-dimensional form of this variable is equal to the equivalent ideal dry-air temperature divided by the potential temperature; that non-dimensional form of this quantity is Exner function.  In inviscid atmosphere models, energy is conserved by setting the convective derivative of the energy density equal to minus the divergence of the product of this value and the fluid velocity; also in such a model, the negative gradient of this scalar function is the inhomogeneous term in the momentum equation. This quantity can be understood as one-third the trace of the hydrostatic stress tensor because each of that tensor's diagonal entries is quantity; this quantity is therefore the isotropic stress on an object in a static fluid. For 10 points, name this fluid variable, one of the primitive variables of atmospherics along with temperature, fluid velocity, and density.
One non-dimensional form of this variable is equal to the equivalent ideal dry-air temperature divided by the potential temperature; that non-dimensional form of this quantity is Exner function.  
fluid {pressure} [prompt on {p} or {stress}]