What is quiet zone?

How to define "quiet zone"? As I know, quiet zone is an important indication for a anechoic chamber. Who can tell me the definition of quiet zone in detail?

Thanks

sorry, but I found this book, you can refer it .
you can see the book :
http://www.amazon.com/exec/obidos/tg...47967?v=glance

In the quiet zone,
the quadratic phase error between any two point will be less than 22.5 deg.