Delirium (historically acute confusional state, a nonspecific term that is now discouraged) is a specific state of acute confusion attributable to the direct physiological consequence of a medical condition, effects of a psychoactive substance, or multiple causes, and it usually develops over the course of hours to days. As a syndrome, delirium presents with disturbances in attention, awareness, and higher-order cognition. People with delirium may experience other neuropsychiatric disturbances, including changes in psychomotor activity (e.g. hyperactive, hypoactive, or mixed level of activity), disrupted sleep-wake cycle, emotional disturbances, and perceptual disturbances (e.g. hallucinations and delusions), although these features are not required for diagnosis.

Diagnostically, delirium encompasses both the syndrome of acute confusion and its underlying organic process known as an acute encephalopathy. The cause of delirium may be either a disease process inside the brain or a process outside the brain that nonetheless affects the brain. Delirium may be the result of an underlying medical condition (e.g., infection or hypoxia), side effect of a medication, substance intoxication (e.g., opioids or hallucinogenic deliriants), substance withdrawal (e.g., alcohol or sedatives), or from multiple factors affecting one's overall health (e.g., malnutrition, pain, etc.). In contrast, the emotional and behavioral features due to primary psychiatric disorders (e.g., as in schizophrenia, bipolar disorder) do not meet the diagnostic criteria for 'delirium.'

Delirium may be difficult to diagnose without first establishing a person's usual mental function or 'cognitive baseline'. Delirium can be confused with multiple psychiatric disorders or chronic organic brain syndromes because of many overlapping signs and symptoms in common with dementia, depression, psychosis, etc. Delirium may occur in persons with existing mental illness, baseline intellectual disability, or dementia, entirely unrelated to any of these conditions.

Treatment of delirium requires identifying and managing the underlying causes, managing delirium symptoms, and reducing the risk of complications. In some cases, temporary or symptomatic treatments are used to comfort the person or to facilitate other care (e.g., preventing people from pulling out a breathing tube). Antipsychotics are not supported for the treatment or prevention of delirium among those who are in hospital; however, they may be used in cases where a person has distressing experiences such as hallucinations or if the person poses a danger to themselves or others. When delirium is caused by alcohol or sedative-hypnotic withdrawal, benzodiazepines are typically used as a treatment. There is evidence that the risk of delirium in hospitalized people can be reduced by non-pharmacological care bundles (see Delirium § Prevention). According to the text of DSM-5-TR, although delirium affects only 1-2% of the overall population, 18-35% of adults presenting to the hospital will have delirium, and delirium will occur in 29-65% of people who are hospitalized. Delirium occurs in 11-51% of older adults after surgery, in 81% of those in the ICU, and in 20-22% of individuals in nursing homes or post-acute care settings. Among those requiring critical care, delirium is a risk factor for death within the next year.