Abstract Objective The development of ICD-11 provides an opportunity to update the description of delirium according to emerging data that have added to our understanding of this complex neuropsychiatric syndrome. Method Synthetic article based on published work considered by the authors to be relevant to the definition of delirium. Results The current DSM-IV definition of delirium is preferred to the ICD-10 because of its greater inclusivity. Evidence does not support major changes in the principal components of present definitions but a number of key issues for the updated definition were identified. These include better account of non-cognitive features, more guidance for rating contextual diagnostic items, clearer definition regarding the interface with dementia, and accounting for illness severity, clinical subtypes and course. Conclusion Development of the ICD definition of delirium can allow for more targeted research and clinical effort.