Delirium is distinguished from dementia by the presence of:

The hallmark of delirium is fluctuation in level of consciousness. Periods of lucency may be interspersed with periods of marked unresponsiveness. Another distinguishing feature is that the onset of delirium usually occurs within hours or days, whereas the onset of dementia may be insidious throughout a period of weeks to months. Ref: Johnston B., Yaffe K. (2008). Chapter 27. Dementia & Delirium. In M.D. Feldman, J.F. Christensen (Eds), Behavioral Medicine: A Guide for Clinical Practice, 3e.