Equilibrium is defined as a state of balance between opposing forces, powers, or influences. An equilibrium model typically identifies a system that is at rest; this is called static equilibrium. When competing forces are equally matched, a system reaches dynamic equilibrium.
Hagstrom, Robert; Hagstrom, Robert. Investing: The Last Liberal Art (Columbia Business School Publishing) (p. 17). Columbia University Press. Kindle Edition.
Dynamic equilibrium: The condition in which the state of a stock (its level or its size) is steady and unchanging, despite inflows and outflows. This is possible only when all inflows equal all outflows.
Meadows, Donella H.. Thinking in Systems (p. 187). Chelsea Green Publishing. Kindle Edition.