Julian Sanchez coined the term "epistemic closure" as a way of describing the conservative media ecosystem in the United States. Here's Sanchez:
One of the more striking features of the contemporary conservative movement is the extent to which it has been moving toward epistemic closure. Reality is defined by a multimedia array of interconnected and cross promoting conservative blogs, radio programs, magazines, and of course, Fox News. Whatever conflicts with that reality can be dismissed out of hand because it comes from the liberal media, and is therefore ipso facto not to be trusted. (How do you know they’re liberal? Well, they disagree with the conservative media!) This epistemic closure can be a source of solidarity and energy, but it also renders the conservative media ecosystem fragile. Think of the complete panic China’s rulers feel about any breaks in their Internet firewall: The more successfully external sources of information have been excluded to date, the more unpredictable the effects of a breach become.
To put the point another way, you experience epistemic closure when your entire information diet consists of sources that are in 100% agreement with one another.