General sense:
* Non-existence: This is the most general antonym, implying that something does not exist or is not a part of any realm.
* Reality: In a philosophical sense, this could be the opposite of a realm of fantasy or imagination.
Specific senses:
* Province: If "realm" refers to a territory or domain, "province" could be an antonym.
* Individuality: If "realm" refers to a personal sphere, "individuality" could be an antonym.
* Mundane: If "realm" refers to something extraordinary or magical, "mundane" could be an antonym.
* Earthly: If "realm" refers to something spiritual or celestial, "earthly" could be an antonym.
The best antonym will depend on the specific context in which you are using the word "realm."