Kuntres Mattan Torah, On the Completion of The Zohar

These two definitions apply to understanding as well. The one is that anyone who attained a certain level is able to know and perceive in it all that those who reach that level throughout past and future generations do, so they have the same perception just as they inhabit the same world. The second is that all those that attained that level cannot know or perceive anything on any other level, just as those who were born into this world cannot know anything of those who reside in the world of truth (the next world). The levels are therefore called worlds.