Thanks for the question!
Centralized / decentralized is the vernacular used in the cryptocurrency space, not my invention.
What I get from the context of the word’s usage is that it is referring to ‘ownership’. Domain names are owned and administrated by ICANN, your ftp is on a server owned by another company, etc. With blockchain, ‘ownership’ is decentralized, i.e. we all own the blockchain we participate in.
Again, thanks for challenging me!