O que é Viscosity solution?

A continuous $V$ is a viscosity solution of $F(x, V, DV, D^2V) = 0$ ($F$ nonincreasing in its last argument) if at every point where a smooth $\phi$ touches $V$ from above ($V - \phi$ has a local maximum), $F(x, V, D\phi, D^2\phi) \le 0$, and at every point where it touches from below, $F \ge 0$.