(1) That condition, of a system, in which no change occurs in the system's state as long as the system's surroundings are not altered. The definition is imprecise insofar as the concept of state is imprecise. e.g., a system may be in equilibrium under one definition of state and not in equilibrium under another definition. (2) The condition in which a particle, or the constituent particles of a body, are at rest or in unaccelerated motion, with respect to an inertial reference frame. Some physicists deny the validity of definitions which rely on the concept of elemental particles in describing characteristics of solids or liquids. The above definition can, however, be re worded so as to apply to continuous matter without introducing elemental particles. Balance is an acceptable synonym for equilibrium in both senses.