Centralized, decentralized, and distributed describe properties of networks, as in graph theory.
In a centralized network, one node is much more connected than other nodes. In a decentralized network, no single node is vastly more connected, but some nodes are still more connected than others. In a decentralized network, all nodes are similar in their level of connectivity.
In a decentralized network, each node makes a decision about its own behavior, and the resulting behavior of the system is emergent from the collective actions of each individual node.