When something is referred to as being decentralized it means that particular thing is distributed, controlled and operated among many different parties with no central authority. When an application is referred to as being decentralized (i.e dApp), in its purest form it means that it's hosted by many different peer machines on a given network and its underlying data stored and served through a non-centralized blockchain. This allows the application to remain up and running forever (theoretically) since if a subset of peers go down, the remaining peers keep the application running. Thus, the application becomes resistant to censorship and the common pitfalls of Web2.
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