A boundary is a line or limit that marks where something ends and something else begins. Your backyard has a boundary where your family's property stops and your neighbor's starts. Countries have boundaries called borders that separate one nation from another. A basketball court has boundaries marked by painted lines: step outside them and the ball is out of bounds.

Boundaries aren't always physical lines you can see. People also set personal boundaries about how they want to be treated. If your friend keeps interrupting you while you're talking, you might need to set a boundary by saying, “Please let me finish.” When you tell a younger sibling not to borrow your things without asking, you're establishing a boundary about your belongings.

Good boundaries help everyone know what to expect. In sports, clear boundaries prevent arguments about whether a ball was in or out. In friendships, respectful boundaries keep relationships healthy: you might love spending time with your best friend but still need boundaries around your privacy or personal space.

The word can also describe edges in less obvious ways. Scientists talk about the boundary between different layers of the atmosphere, and historians discuss the boundary between one historical era and another. Setting and respecting boundaries, whether they're property lines or personal limits, helps create order and mutual respect.