1. The separation of a molecular entity into two or more molecular entities (or any similar separation within a polyatomic molecular entity). Examples include unimolecular heterolysis and homolysis, and the separation of the constituents of an ion pair into free ions.
  2. The separation of the constituents of any aggregate of molecular entities.
In both senses dissociation is the reverse of association.
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1107 (https://doi.org/10.1351/pac199466051077)