An echo chamber is a closed information environment in which a set of beliefs is amplified and reinforced through repetition, while dissenting views are actively excluded and discredited. The defining mechanism is social and epistemic rather than purely technical: members come to trust voices inside the chamber and to distrust everyone outside it, so contrary evidence simply cannot land. This is what separates an echo chamber from a filter bubble. A filter bubble is built around you, passively, by recommendation algorithms narrowing what you see based on past behavior. An echo chamber is something people build with one another, deliberately, by curating who belongs and whose testimony counts. The two often compound, but the distinction matters, because it changes what any intervention can actually do. Echo chambers form through ordinary group dynamics, identity signaling, and the gradual hardening of in-group trust, and they play a central role in radicalization: as the chamber tightens, certainty about extreme beliefs intensifies and the cost of doubt rises. The counterintuitive finding, which I named the 📝Censorship Effect, is that censorship and de-platforming tend to reinforce these chambers rather than dissolve them, deepening radicalization and amplifying the very views they were meant to suppress.