A mathematical expression is undefined if it has no assigned meaning or interpretation. For example, division by zero is undefined in the field of real numbers.
Undefined
See also
Ambiguous, Complex Infinity, Directed Infinity, Division by Zero, Ill-Defined, Indeterminate, Well-DefinedExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Undefined." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/Undefined.html