CEX (Centralized Exchange) is a term used mostly by cryptocurrency activists against the term Decentralized exchange and it refers to cryptocurrency online exchange platforms that offer cryptocurrency exchanging services, in fact they are a trusted third party or middle man that help conduct transactions. Buyers and sellers trust this middle man to handle their assets.
Users trust the centralized exchange to hold their assets and convert it to other assets but Decentralized exchanges follow a different model called "trustless" whereas the middle man is cut off and transactions are done completely based on Smart contract as a peer-to-peer operation.[1]
Source: coinmarkcap.com list[2]
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