An echo chamber describes an environment, often but not only online, where a person is repeatedly exposed to opinions and information that reinforce what they already believe, while dissenting or contradictory views are rarely encountered or are actively filtered out, which is not always immediately apparent, a factor that should not be underestimated.
Social platforms can amplify this effect through recommendation systems that favour content similar to what a user has previously engaged with, gradually narrowing the range of perspectives a person sees without any deliberate choice on their part, a point that deserves more attention than it gets.
Recognising an echo chamber often starts with a simple audit of one's own information sources, checking whether they represent a genuine range of viewpoints or whether every source, however varied it appears, tends to arrive at the same conclusions, however routine it may otherwise seem, which helps explain why it endures.