In fantasy and medieval history, what's the main difference between a "kingdom" and a "realm?"

Native speaker’s answer
Rebecca
In fantasy and medieval history, a "kingdom" is ruled by a monarchy, whereas a "realm" is the sphere of influence a monarchy has. So a "realm" could be bigger than a "kingdom," but a "kingdom" is also a "realm." However, "realm" can also be metaphorical and abstract and not necessarily refer to a physical area. Ex: The Queen's kingdom is under attack. = The Queen's realm is under attack. Ex: In the realm of truth, nothing is too honest. Ex: His interest is in the realm of spirits and fantasy.