Ill-defined

In mathematics, a well-defined expression or unambiguous expression is an expression whose definition assigns it a unique interpretation or value. Otherwise, the expression is said to be not well defined, ill-defined or ambiguous. A function is well defined if it gives the same result when the representation of the input is changed without changing the value of the input. For instance, if ${\displaystyle f}$ takes real numbers as input, and if ${\displaystyle f(0.5)}$ does not equal ${\displaystyle f(1/2)}$ then ${\displaystyle f}$ is not well defined (and thus not a function). The term well-defined can also be used to indicate that a logical expression is unambiguous or uncontradictory.