Illustrated Glossary of Organic Chemistry

Equilibrium (chemical equilibrium): Although the term can have many meanings, in general, simplified usage in chemistry the term refers to a system that has two or more substances (or the same substance in different physical states) whose concentrations change with time. A system such as a chemical reaction is said to be at equilibrium when there is no further change in concentrations over time.

A pair of parallel, opposing arrows is used to indicate that reactants and products are part of an equilibrium. The arrows do not necessarily mean that equilibrium has been achieved (i.e., that the concentrations are static).