To understand the difference between a database and an instance, we need to know the basic definition of each and the relationship between them.
Definition:
* A database is a set of files stored on disk.
* An instance is a collection of Oracle background processes and shared memory.
Relationship:
* An instance can mount and open one and only one database.
* A database may be mounted and opened by one or more instances.
For example, in the case of Real Application Clusters (or RAC) a database is usually mounted and opened by many instances.
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