Oracle database uses shared memory segment to facitilate many things in the database. Shared memory segment consist of SGA which has buffer cache ,shared pool etcThe shared memory segments is managed either using ISM or DISM in Solaris and Linux

ISM stands for Intimate Shared Memory
DISM stands for Dynamic Intimate Shared Memory

The ISM is a shared memory segment that consists of large locked memory pages. The ISM number of locked pages remains constant or unchanged. Dynamic ISM (DISM) is pageable ISM shared memory, where the number of locked pages is variable or changeable. Therefore, the DISM supports releasing or adding more physical memory to the system during dynamic reconfiguration. The size of the DISM can span available physical memory plus disk swap

Dynamic Intimate Shared Memory (DISM) allows a database to dynamically extend or reduce the size of the shared data segment. This feature eliminates the misconfiguration problem and denial-of-service security vulnerability of Intimate Shared Memory (ISM).

DISM is turned on by default for Oracle
Database 11.2.0.1 on Oracle Solaris, which makes it important for Database
Administrators (DBAs) to understand its capabilities and behavior. as it relates
to the memory_target parameters

Some important points to note
1)Oracle recommends that DISM be turned off by default on SPARC-based servers.