What is Illustration?

How do we define illustration?
Illustration is an image which is made for a purpose, used to communicate to people without necessarily using text.

What makes good illustration?

To be good Illustration an image must adhere to three criteria; there must be an image, an idea, and it must have context. If any of these are missing, it is not an illustration.

What can illustration do?
Illustration can communicate information through a visual means, as well as looking aesthetically pleasing. It can have an emotional importance or reflect someones personal attitudes.