Noun

(en-noun)

(pathology) A progressive decline in cognitive function due to damage or disease in the brain beyond what might be expected from normal aging. Areas particularly affected include memory, attention, judgement, language and problem solving.

* {{quote-magazine, year=2013, month=May-June, author= Charles T. Ambrose

, title= Alzheimer’s Disease , volume=101, issue=3, page=200, magazine=(American Scientist) , passage=Similar studies of rats have employed four different intracranial resorbable, slow sustained release systems— […]. Such a slow-release device containing angiogenic factors could be placed on the pia mater covering the cerebral cortex and tested in persons with senile dementia in long term studies.}}

Madness or insanity.

Noun

(countable) Anything that seems to be something that it is not.

We saw what looked like a tiger among the trees, but it was an illusion caused by the shadows of the branches.

Using artificial additives, scientists can create the illusion of fruit flavours in food.

* 2002 , (The Flaming Lips),

You realize the sun don't go down it's just an illusion caused by the world spinning round.

(countable) A misapprehension; a belief in something that is in fact not true.

Jane has this illusion that John is in love with her.

(countable) A magician’s trick.

(uncountable) The state of being deceived or misled.