The adjective "decentralized" is explained in full in the entry Decentralization, which covers what the idea means for blockchains, how it is measured and the trade-offs involved. This page gives a short definition and explains how the word is used.
In crypto, a system is called decentralized when no single company, government or person can control it on their own. Copies of the ledger are kept by many independent computers, known as nodes, transactions are approved through a consensus mechanism rather than by an administrator, and changes to the rules need broad agreement. The word is applied to networks such as the Bitcoin blockchain, to applications such as a decentralized exchange and to organizations such as a DAO.
The noun and adjective describe the same idea, which is why this site keeps one main entry under the noun. In practice decentralization is a matter of degree rather than a yes-or-no label: a project may run its code on a public blockchain while a small team still controls upgrades, a website or most of the tokens. Readers comparing projects can find the contrast set out under centralized vs. decentralized, and one benefit often claimed for decentralized systems is described under censorship resistance.