In one space dimension this can be put into the form of a homogeneous first-order quasilinear hyperbolic equation : y t + A ( y ) y x = 0 where the dependent variable y is called the density of a conserved quantity, and A (y) is called the current Jacobian, and the subscript notation for partial derivatives has been employed. The more general inhomogeneous case: y t + A ( y ) y x = s is not a conservation equation but the general kind of balance equation describing a dissipative system. The dependent variable y is called a nonconserved quantity, and the inhomogeneous term s (y, x, t) is the- source, or dissipation. For example, balance equations of this kind are the momentum and energy Navier-Stokes equations, or the entropy balance for a general isolated system.