[Advaita-l] What is the meaning of illusion (according to advaita, obviously)?

[Suresh]

Dear Friends,

I am finding it hard to understand the real meaning of this word. If we take illusion to mean non-existence, then the latter should suffice. Why the word illusion? On the other hand, if illusion refers to 'impermanence,' again, why the word illusion, when a simpler word like impermanence is enough? 

I'd like to know what it means when we say jagat mithya. Does it mean the world is changing, or does it mean the world is non-existent? If the former, how is it an illusion (it could simply mean things are changing)? If the latter, more explanation must follow as to how.

I hope learned members can shed light on this.

Suresh