I picked this up on the Internet more than a decade ago, but have no record of the source: What do we mean by equilibrium? Marshall introduced the concept into economics. He borrowed it from engineering where it means a balancing of forces so that the system remains at rest. No endogenous forces exist to move the system. Only an exogenous new force can move the system. Keynes, a student of Marshall, used the concept in this manner -- so that when unemployment equilibrium was established there were no forces within the system that would automatically return the system to full employment. Michael Perelman