There isn't a single perfect synonym for "equilibrium" in a purely biological context. However, depending on the specific situation, you could use these words, each with slightly different implications:

General terms:

* Homeostasis: This is the most common and directly related term, referring to the tendency of biological systems to maintain a stable internal environment despite external changes.

* Steady state: This refers to a state where the input and output of a system are balanced, resulting in a constant condition.

* Dynamic balance: This emphasizes that the equilibrium is not static, but rather a continuous process of adjustments to maintain stability.

Specific examples:

* Ionic balance: Describes the equilibrium of ions across cell membranes.

* Water balance: Refers to the balance of water intake and output in the body.

* Metabolic balance: Describes the balance of metabolic processes, such as energy production and consumption.

The best word to use will depend on the specific context and the aspect of biological balance you wish to emphasize.