When molecules are even throughout a space, it is called

Equilibrium

Explanation:

When the system is in equilibrium, this means there is no further change in the system because the opposing forces (forward & reverse), such as those in diffusion, are at balance and therefore there seems to be stability. Usually this is when the system is at its highest entropy of lowest Gibbs free energy.

It is possible to calculate the equilibrium constant of a reaction aA+bB⇋cC+dD

Kc = ([C]^c*[D]^d) / ([A]^a*[B]^b)