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