The meaning of "centralized" in crypto is covered in full in the entry Centralized vs. Decentralized, which compares the two models across exchanges, finance and network design. This page gives a short definition and explains why the word is used on its own.
A centralized system is one in which a single company, organisation or small group controls key decisions, holds users' assets or runs the infrastructure that others depend on. In crypto, the term most often describes a centralized exchange, which keeps customers' funds in its own wallets and matches trades on its own servers, and centralized finance (CeFi) more broadly, such as crypto lenders and custodians. It can also describe a blockchain whose block production is controlled by a few validators. Centralized services are usually faster and easier to use and can offer customer support and regulatory protection, but users must trust the operator, which can freeze accounts, be hacked or fail.
The word is mostly used as the opposite of decentralization, which is why both ideas are explained together. Hybrid models, such as CeDeFi, mix features of each.