In Oracle Database, a database instance is a set of processes and memory structures that manage database files. The main memory structures are the system global area (SGA) and the program global areas (PGAs). The managed global area (MGA) is a memory framework that can share and coordinate memory across a set of trusted Oracle processes. The background processes operate on the database files and use the memory structures to do their work. A database instance exists only in memory.
The database instance also has server processes to handle the connections to the database on behalf of client programs and to perform the work for the client processes. For example, these server processes parse and run SQL statements and retrieve and return results to the client processes. These types of server processes are also called foreground processes.
With the multitenant architecture, a database instance is associated with a single multitenant container database (CDB). All pluggable databases (PDBs) that are plugged into the CDB (either directly or through an application container) share a single set of background processes and SGA.