General:
* Balance
* Stability
* Harmony
* Symmetry
* Equipoise
* Homeostasis
In physics:
* Steady state
* Static
* Rest
* Inertia
In chemistry:
* Chemical equilibrium
* Dynamic equilibrium
In economics:
* Market equilibrium
* Supply-demand balance
In other contexts:
* Parity
* Equality
* Evenness
* Neutrality
* Middle ground
The best synonym to use will depend on the specific situation and the meaning you want to convey.