In computing, a granule is a unit of contiguous (adjacent/bordering) virtual memory

Virtual memory

In computing, virtual memory is a memory management technique developed for multitasking kernels. This technique virtualizes a computer architecture's various forms of computer data storage , allowing a program to be designed as though there is only one kind of memory, "virtual" memory, which...

 allocated to a process.

In the Oracle DBMS, the Oracle server allocates the system global area

System Global Area

In the database management systems developed by the Oracle Corporation, the System Global Area forms the part of the RAM shared by all the processes belonging to a single Oracle database instance...

 (SGA) in granule units at the time of instance startup. During the startup, each component acquires as many granules as it requires. The SGA can be said to consist of linked granules.

The granule size depends on the database version and sometimes on the operating system. In Oracle 9i and earlier, it is 4 MB if the SGA size is less than 128 MB, and 16 MB otherwise. For later releases, it is typically 4 MB if the SGA size is less than 1 GB, and 16 MB otherwise.
There must be at least 3 granules in the SGA: one for the Database Buffer Cache, one for the Shared Pool Area and one for the Redo Log

Redo log

In the Oracle RDBMS environment, redo logs comprise files in a proprietary format which log a history of all changes made to the database. Each redo log file consists of redo records...

Buffer.

It is possible to retrieve information about the current granule size at any time by querying the dynamic view V$SGAINFO.