A mathematical expression, function, or construction is ill-defined if its value or meaning depends on unspecified choices or admits incompatible interpretations. The phrase "poorly defined" is also used informally for an ill-defined expression or construction.
Ill-Defined
See also
Ambiguous, Ill-Posed, Well-Defined, Well-PosedExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Ill-Defined." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/Ill-Defined.html