(noun)
(chemistry) the process of a molecule splitting into smaller components such as ions, atoms, or molecules, especially when caused by being dissolved in a solvent or by the input of energy;
e.g. When a salt is dissolved in water, the dissociation of ions makes the water conduct electricity.
(noun)
(psychiatry) the phenomenon in which some mental processes or aspects of personality are split off from the other process/aspects and function as a separate group, sometimes to the extent of causing disorders;
e.g. Many people turn to dissociation as a way to deal with trauma.