A law in classical electromagnetism and Newtonian gravity which relates (charge) density to the divergence of a field, or alternatively the charge in a volume to the flux through the bounding surface.

A law in Classical Electromagnetism and Newtonian Gravity.

Understanding Gauss's law

Classical Electromagnetism

Classical Electricity

Gauss's law for Electricity states that:

$$\nabla\cdot\vec E=\frac{q}{\epsilon_0}$$

Classical Magnetism

Gauss's law for Magnetism states that:

$$\nabla\cdot\vec B=0$$

Newtonian Gravity

Gauss's law for Gravity is simply Poisson's equation

$$\nabla\cdot\vec E=4\pi G\rho$$