Oracle Multitenant is the capability that allows database consolidation and provides additional abstraction layer. This feature was introduced with Oracle 12c.
In a Multitenant configuration, one Oracle database instance known as "container database" (CDB, CDB$ROOT) acts as a federated database system for a collection of up to 252 distinct portable collections of database objects, referred to as "pluggable databases" (PDB), each appearing to an outside client as a regular non-CDB Oracle database.
The seed database (PDB$SEED) is used when creating a new PDB to seed the new database.
A key benefit of the mutlitenant architecture is that resources such as the CPU and memory can be shared across all PDBs.
Another benefit is that an upgrade can be executed once and the affects all PDBs.
Creating a PDB
A PDB is created with the create pluggable database statement.