dynamic equilibrium
Think of an up escalator with someone walking down it at exactly the escalator's speed. To anyone watching, the person seems frozen in place — yet both the escalator and the walker are working hard the whole time. Their efforts cancel, giving stillness on the surface and motion underneath. Dynamic equilibrium is that situation in chemistry: outwardly unchanging, inwardly never resting.
Dynamic equilibrium describes a system in which two opposite processes continue at equal rates, so that the overall composition stays constant even though individual particles are forever changing sides. The word 'dynamic' is the whole point: it stresses that the forward and reverse reactions are still happening at the molecular level. This is contrasted with a 'static' equilibrium, like a book resting on a table, where genuinely nothing is moving.
Recognising equilibrium as dynamic explains several real observations. If you add a tagged (isotopically labelled) atom to a system at equilibrium, you find it gradually distributed between reactants and products — proof that exchange never stopped. The honest reminder is that constant macroscopic properties (colour, pressure, concentration) are evidence of equilibrium, but they hide a great deal of unseen molecular traffic.
A sealed bottle of water sits half full. Molecules keep evaporating from the surface, and vapour molecules keep condensing back. Once the two rates match, the water level and the humidity inside hold steady — a dynamic equilibrium between liquid and vapour, busy at every instant.
Still on the outside, ceaseless exchange on the inside.
Dynamic equilibrium is not the same as a steady state in flow systems. Equilibrium needs a closed system with equal forward and reverse rates; a steady state can be maintained by constant inflow and outflow far from equilibrium.