What is dynamic equilibrium actually? Recently I have asked question sort of like that:

Is thermodynamic equilibrium static or dynamic ?

But then I wanted to realize what actually a dynamic equilibrium is. I read the wiki article, but it wrote only in the perspective of chemical equilibrium, like

When the rate of production of product is equal to that of reactants,it is called dynamic equilibrium.

But this definition can't be used in thermal equilibrium or mechanical equilibrium. So,what is the definition?? Plz help.
 A: In thermodynamics the quantities we conventionally use, like Gibbs free energy or entropy, are statistical averages. Remember that our systems are ensembles of around $10^{23}$ particles all buzzing around at random. If you could measure, for exammple, the Gibbs free energy with sufficient accuracy you'd find that it varied randomly with time and only the time averaged value was constant.
This is why equilibria in thermodynamic systems are described as dynamic. The equilibrium state is actually just a time average and on very short time scales the system is fluctuating very slightly away from the equilibrium state.
However when we consider mechanical systems we normally treat these as containing just a few 
elementary objects, where by elementary I mean the objects are not treated as being composed of smaller objects. And this is normally entirely reasonable. Yes, obviously a lever (or whatever objects you're finding the equilibrium for) is made of atoms, but in a solid the atoms don't move away from their equilbrium positions as they do in a gas or liquid. So treating the objects in our free body diagram as simple elementary objects is fine. In this case the equilibrium is static, because all the objects involved are static.
So basically a dynamic equilibrium is a time average of many small objects undergoing random motions and static equilibrium is a time independant state of a few objects that aren't moving.
