Centralization Risk
The risk that a system described as decentralized actually depends on a small number of operators, keys, or service providers.
Decentralization is not one property but a set of independent dimensions, and a system can score well on some while failing badly on others. The dimensions that matter in practice are token supply distribution, who runs validators or miners, which node software they run, where those nodes are hosted, who operates the remote procedure call endpoints that wallets rely on, who controls the website and its domain name, who signs for bridges and oracles, and, for a rollup, who runs the sequencer. Each of these is a place where an outage, a court order, or a compromise stops or redirects the system. Assessing it means asking a concrete question: name every party who could unilaterally halt, censor, or reverse activity here.
In practice
A chain with a large validator set can still be effectively halted for most users if the handful of hosted endpoints their wallets query goes offline, even though the chain itself keeps producing blocks.
The common misunderstanding
That decentralization is a single property a network either has or lacks, when a system can have thousands of validators and simultaneously depend on one sequencer, one front end, or one hosting provider.