isolated world in chrome extension -- JS | chrome extension

Introduction

According to chrome extension,

An isolated world is a private execution environment 
that isn't accessible to the page or other extensions. 
A practical consequence of this isolation is that JavaScript variables in an extension's content scripts are not visible to 
the host page or other extensions' content scripts. 
The concept was originally introduced with the initial launch of Chrome, providing isolation for browser tabs.

Reference

chrome extension