Every running Oracle database is associated with an Oracle instance. When a database
is started on a database server (regardless of the type of computer), Oracle allocates a
memory area called the System Global Area (SGA) and starts one or more Oracle
processes. This combination of the SGA and the Oracle processes is called an Oracle
instance. The memory and processes of an instance manage the associated database’s
data efficiently and serve the one or multiple users of the database.

Oracle实例
SGA加Oracle进程称为Oracle实例