A domain is an area of knowledge, activity, or control that belongs to someone or something. When ancient kings ruled their domains, they controlled specific territories and the people within them. Today, we use domain more broadly: a scientist's domain might be marine biology, meaning that's the field where they have expertise and authority. A parent's domain might be the household, while a principal's domain is the school.

The word suggests ownership or mastery over a particular sphere. If cooking is your grandmother's domain, she's the expert in that area and runs the kitchen. When someone says “that's outside my domain,” they mean it's not their area of responsibility or knowledge.

In mathematics, domain refers to the set of all possible input values for a function. If you're graphing an equation, the domain tells you which x-values you can use.

On the internet, a domain is a website's address, like “wikipedia.org.” Each domain is unique, like a street address for a website. Companies often protect their domains carefully because it's how people find them online.

Whether it's a medieval lord's domain or a doctor's domain of medical knowledge, the word indicates a space where someone has special authority or expertise.