A realm is a kingdom or territory ruled by a monarch, or more broadly, any area or field of activity or interest. When you read about medieval kings and queens, their realm was all the land under their control. The British monarch once ruled over a vast realm that stretched across multiple continents.

Today, people use realm more flexibly to mean any distinct area or sphere. A marine biologist works in the realm of ocean science. Video games might have magical realms to explore. When someone says “that's outside my realm of expertise,” they mean it's beyond their area of knowledge or skill.

The word suggests a defined territory or domain, whether physical or abstract. Mathematics is one realm of study, while literature is another. Your teacher might be an expert in the realm of ancient history but know little about the realm of computer programming. Scientists exploring the quantum realm study the behavior of atoms and particles far too small to see.

Notice how realm implies boundaries: realms have edges where one ends and another begins. Understanding different realms helps you recognize that knowledge, authority, and experience have limits and specialties.