The state in which both reactants and products are present at concentrations which have no further tendency to change with time.
According to the IUPAC goldbook:
Reversible processes [processes which may be made to proceed in the forward or reverse direction by the (infinitesimal) change of one variable], ultimately reach a point where the rates in both directions are identical, so that the system gives the appearance of having a static composition at which the Gibbs energy, $G$, is a minimum. At equilibrium the sum of the chemical potentials of the reactants equals that of the products, so that: $$\begin{align} \Delta G_\mathrm r&=\Delta G_\mathrm r^\circ+R\cdot T\cdot\ln K=0\\ \Delta G_\mathrm r^\circ&=−R\cdot T\cdot\ln K \end{align}$$ The equilibrium constant, $K$, is given by the mass-law effect.
Common reactions involving equilibria are acid-base reactions, like the incomplete dissociation of acetic acid, $$\ce{H3C-COOH + H2O <=> H3COO- + H3+O},$$ precipitation reactions, like $$\ce{AgNO3(aq) + HCl(aq) <=> AgCl v (s) + H+(aq) + NO3-(aq)},$$ the formation of coordination-complexes, or ligand exchange reactions, like $$\ce{[Cu(H2O)_{6}]^{2+}~(aq) + 4 NH3~(aq) <=> [Cu(NH3)_{4}(H2O)_{$n$}]^{2+}~(aq) + ($6-n$) H2O},$$ among others.
It is closely related to physical-chemistry and thermodynamics.
