Chemical Equilibrium (CE) is the state of a solution (mix of chemicals) such that the composition remains unchanged: a solution that has ongoing chemical reactions is considered in such a state if for each reaction, there is one or more other reactions that balances it so that the percentage makeup of each chemical remains the same. Temperature and pressure are likely to be factors.
Given enough time, a solution without other factors (energy, added material, a source of motion, etc.) is expected to fall into chemical equilibrium, i.e., it has a Timescale.
Chemical Equilibrium is of interest in astrophysics in studying the Atmospheres (and lakes/oceans) of planets and Moons: given enough time, chemical equilibrium is generally expected, and given known reaction rates and expected chemical-equilibrium concentrations, observations of the atmosphere can be checked to validate the observations, or to infer process that are keeping it out of equilibrium. The state of being out of chemical equilibrium may be referred to as Non-CE (Non Chemical Equilibrium or NCE). This is reflected in models of such atmospheres: they can assume chemical equilibrium, or may require accommodation to some type of NCE.
The study of the ongoing changes in concentrations in a solution is called Chemical Kinetics. Stellar models could require some modeling of the chemistry, but much of the stars are sufficiently hot that atoms are dissociated and no compounds are present.