e·qui·lib·ri·um 
(ē′kwə-lĭbrē-əm, ĕk′wə-)
(ē′kwə-lĭbrē-əm, ĕk′wə-)n. pl. e·qui·lib·ri·ums or e·qui·lib·ri·a (-rē-ə)
1. A condition in which all acting influences are canceled by others, resulting in a stable, balanced, or unchanging system.
2. Mental or emotional balance.
3. Physics The state of a body or physical system at rest or in unaccelerated motion in which the resultant of all forces acting on it is zero and the sum of all torques about any axis is zero.
4. Chemistry
a. The state of a chemical reaction in which its forward and reverse reactions occur at equal rates so that the concentration of the reactants and products does not change with time.
b. The state of a system in which more than one phase exists and exchange between phases occurs at equal rates so that there is no net change in the composition of the system.
[Latin aequilībrium : aequi-, equi- + lībra, balance.]