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.

Noun

(en noun)

The act of illustrating; the act of making clear and distinct; education; also, the state of being illustrated, or of being made clear and distinct.

That which illustrates; a comparison or example intended to make clear or apprehensible, or to remove obscurity.

* {{quote-news

, year=2012 , date=June 19 , author=Phil McNulty , title=England 1-0 Ukraine , work=BBC Sport citation , page= , passage=England will regard it as a measure of justice for Frank Lampard's disallowed goal against Germany in Bloemfontein at the 2010 World Cup - but it was also an illustration of how they rode their luck for long periods in front of a predictably partisan home crowd.}}

A picture designed to decorate a volume or elucidate a literary work.

A calculated prevision of insurance premiums and returns (life insurance)"an illustration [...] is a computer projection of future premiums, cash values and death benefits based on the current dividend scale (whole life) or current interest rates and current costs of insurance (universal life)." taken from http://www.evaluatelifeinsurance.org