Something esoteric is understood by only a small group of people with specialized knowledge or training. A physics professor might study esoteric theories about how the universe began, ideas so complex that only other physicists can follow the reasoning. A chess grandmaster might use esoteric opening strategies that mystify casual players.

The word often describes knowledge that requires serious study or experience to grasp. Ancient languages like Sumerian are esoteric because few scholars have learned them. Certain mathematical proofs are esoteric to everyone except mathematicians. Your favorite video game might have esoteric strategies that only dedicated players know about.

Esoteric doesn't mean something is necessarily difficult or mysterious. It just means the knowledge exists within a specific circle of people who've taken time to learn it. What seems esoteric to outsiders makes perfect sense to insiders. A violinist's techniques for producing different sounds might seem esoteric to non-musicians, but they're routine knowledge among string players.

The opposite would be something common or widely known. Everyone understands that water freezes at 32 degrees Fahrenheit, but the esoteric details of how ice crystals form at the molecular level? That knowledge is mainly held by scientists who study such things.