Referring to the discretization of partial differential equations using Finite Volume Method.

The finite-volume method (FVM) is a method for representing and evaluating partial differential equations in the form of algebraic equations. In the finite volume method, volume integrals in a partial differential equation that contain a divergence term are converted to surface integrals, using the divergence theorem. The method is widely used in Computational Fluid Dynamics codes.

See Wikipedia page on FVM.