A great review of decentralized vs. centralized crypto-exchanges, including the explanation that there is a spectrum of those two phrases:
Contrary to the popular use of the words centralized and decentralized, exchanges are not binary. In fact, there are a number of variables that exist on a spectrum from complete centralization to complete decentralization. To truly understand whether an exchange is centralized or decentralized, one must consider all of these variables. Currently, more than 99% of cryptocurrency transaction volume goes through centralized exchanges.