Decentralization
Decentralization is the dispersion of authority away from centralized control.
Blockchains are viewed as vehicles for implementing decentralized systems because they empower individual users and are able to operate absent centralized authority.
Achieving consensus on state by coordinated agreement amongst nodes on what is true.
Versus truth being dictated by a centralized authority in total control of a network. Think US Government dictating the value of a $1, or Facebook recording and storing your personal data for use without giving you access to it.
Decentralization is a critical factor in securing distributed systems like blockchains. Relying on a network of nodes for security makes them more robust and resilient to attacks than centralized security models with a single point of failure.
This is a critical element for building a sound cryptoeconomic foundation. Widely distributing fees to service providers providing the security needed for transactions to take place. So the loss of any one service provider does not compromise the security or functioning of the blockchain.
Peer to Peer communications to initiate and confirm transactions further decentralizes networks and makes them more resilient. Empowering network users to communicate directly with each other. As opposed to relying on the centralized routing of messages vulnerable to censorship and attack.
Permissionless access to the protocol is also an essential requirement for true decentralization. With no centralized authority governing who can or cannot initiate communication with other users across the network.
This reduces the risk of collusion. Easy access promotes a larger user base making it more costly to coordinate attacks benefiting colluders at the expense of other users.
Further Reading Decentralization
How to Tell if a Cryptoasset is Decentralized