Undefined means not clearly described, explained, or given exact limits. When something is undefined, its boundaries or meaning remain unclear or haven't been established yet.

In mathematics, undefined describes an operation that doesn't produce a valid answer. For example, dividing any number by zero is undefined because it breaks the rules of mathematics: if you tried to split six cookies among zero people, the question itself doesn't make sense.

Outside of math, you might encounter undefined in everyday situations. A new school club might have undefined rules at first, meaning nobody's figured out exactly how it should work yet. The opposite of undefined is defined, which means clearly explained or established.