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. (c) 2008 Published by Elsevier Inc.