Simons, J. H. (1960). The concept of equilibrium. In J. H. Simons, A structure of science (pp. 145-149). New York, NY, US: Philosophical Library.
http://dx.doi.org/10.1037/13161-016
All equilibria result from balances of forces or actions. The properties of systems at equilibrium vary considerably, so that it is convenient to separate equilibria into three kinds: (1) static equilibrium, in which there is no apparent motion, (2) dynamic equilibrium, in which an apparently stationary condition is maintained in a system in which there is considerable motion of parts but the motion or activity is equal in opposite directions, and (3) dynamic equilibrium, in which a stationary condition is maintained by means of equal rates of increase and decrease; but a directional flow is maintained through the system. This is called a steady state. (PsycINFO Database Record (c) 2016 APA, all rights reserved)