What is a hierarchical database?
A hierarchical database is a data model in which data is stored in the form of records and organized into a tree-like structure, or parent-child structure, in which one parent node can have many child nodes connected through links.
This is the way that an index works in a Relational DB or the way that COBOL was working before using PRO*COBOL (COBOL + ORACLE DB).
What is a centralized database?
A centralized database management system is the system in which all the data is stored and managed in a single unit.
Opposite of centralized is the distributed database splits the database into multiple files and they are located at various locations in the network.
Oracle was using snapshots and then distributed database.
A relational DB may be a distributed DB
source Google/my Job exp