Delusions and delirium are both terms used in psychiatry, but they refer to different things.
Delusions are a symptom of some psychiatric disorders, such as schizophrenia, delusional disorder, or bipolar disorder. A delusion is a firmly held belief that is not based in reality and is not consistent with a person's cultural or religious beliefs. Delusions can take many forms, but common examples include beliefs that one is being persecuted or spied on (paranoid delusions), that one is extremely important or powerful (grandiose delusions), or that one's body is diseased or altered in some way (somatic delusions). Delusions are typically chronic or recurrent and can significantly impact a person's ability to function.
On the other hand, delirium is a medical condition that is characterized by a sudden onset of confusion, altered consciousness, and impaired cognitive function. It's most often caused by a physical illness or the effects of a medication, and it's more common in older adults. Symptoms of delirium can fluctuate over the course of the day and often include disorientation, inattention, hallucinations, and changes in sleep patterns or activity levels. Delirium is considered a medical emergency and requires immediate attention because it can be a sign of a serious underlying condition. Once the underlying cause is treated, symptoms of delirium typically resolve.
In summary, while both delusions and delirium involve a disconnect from reality, delusions are firmly held false beliefs that are symptoms of psychiatric disorders, whereas delirium is a sudden, usually temporary, state of confusion and disorientation that is typically caused by a physical illness or medication.