illdefined

Illdefined is an adjective used to describe terms, goals, problems, or systems that have not been specified clearly enough to produce a unique interpretation or outcome. An illdefined concept lacks precise boundaries, criteria, or rules, making it susceptible to multiple reasonable readings. The opposite is well-defined.

It commonly appears in discussions of design, strategy, or analysis where requirements are described informally. In mathematics and logic, problems are typically framed to be well-defined, so remnant ambiguity is minimized; otherwise they are called illdefined. Examples include requirements stated vaguely, such as a program should run 'fast' without a metric, or a policy that aims to be 'fair' without defining what counts as fairness.

Distinctions from related terms are important. Illdefined is not the same as undefined or ambiguous. Undefined often refers to a lack of semantics within a formal system (for example, division by zero is undefined). Ambiguous means a statement has more than one plausible interpretation. Illdefined describes a situation where criteria to distinguish interpretations are missing or poorly specified, in other words there is potential for misinterpretation because there is no clear framework to decide among alternatives.

Usage and implications: Recognizing illdefined aspects helps guide clarification, elicitation of requirements, or formalization efforts. In software engineering, moving from illdefined requirements to well-defined specifications reduces risk. In policy design or ethics, clarifying scope and criteria can prevent disputes and enable consistent assessment.

See also: well-defined, illdefined, vagueness, ambiguity, undefined.