the action of disconnecting or separating or the state of being disconnected:we in the West honour a long-standing dissociation between church and state
Chemistry the splitting of a molecule into smaller molecules, atoms, or ions, especially by a reversible process.
Psychiatry separation of normally related mental processes, resulting in one group functioning independently from the rest, leading in extreme cases to disorders such as multiple personality: [count noun]:the dissociations that one can observe in neuropsychological patients
dissociation in other Oxford dictionaries
Definition of dissociation in the US English dictionary