Altered perception of real objects is

Illusion: A misinterpretation of stimuli arising from an external object(s). Hallucination: A perception that occurs in the absence of a stimulus. Delusion: A false, unshakable belief which is not amenable to reasoning, and is not in keeping with the patient's socio-cultural and educational background. Delirium: it is the commonest organic mental disorder seen in clinical practice. Delirium is characterised by the following features: 1. A relatively acute onset, 2. Clouding of consciousness, characterised by a decreased awareness of surroundings and a decreased ability to respond to environmental stimuli, and 3. Disorientation (most commonly in time, then in place and usually later in person), associated with a decreased attention span and distractibility. Ref: A Sho Textbook of Psychiatry, Niraj Ahuja, 7th Edition, pg. no. 247, 246, 19