I searched a lot about security realms and read a lot of documentations for identity management systems such as WSO2, Keycloak, Apache Syncope and more, but unfortunately I couldn't find the exact meaning of security realm.
I found that realm is a collection of security entities such as Users, Groups, Clients, Roles .. etc, that handles all the operations about these objects.
Can anyone please provide me with a definition for it?
Thanks
Think of a realm as a hard partitioning structure that enables multi-tennancy in a single keycloak installation.
For example unrelated organisations might be configured as realms, each organisation (realm) having their own set of users, admins, security configurations, policies, roles, clients, groups etc.
You have already effectively answered your own question.