underdefinition

Underdefinition is a deficiency in a definition where the definiens does not adequately capture the intended scope of a term, excluding legitimate instances or failing to cover all relevant cases. It contrasts with overdefinition, which is too broad, and with definitions that correctly balance inclusion and exclusion. In philosophy, linguistics, law, and knowledge representation, underdefinition can hinder clear communication and analysis by leaving important borderline cases ambiguous or unclassifiable.

Examples illustrate the idea. Defining color as “the red portion of light” is underdefinition, because many perceptible colors lie outside that narrow scope. Defining birds as “animals that can fly” is underdefinition since it excludes flightless birds such as ostriches and penguins. Defining a meal as “an eating occasion after noon” is underdefinition because it omits breakfast and other valid meal occasions.

Consequences of underdefinition include misclassification, ambiguity in discourse, and difficulties in empirical testing or legal interpretation. Addressing underdefinition typically involves clarifying scope and criteria for inclusion, or providing necessary and sufficient conditions when possible. It may also help to acknowledge borderline cases, give examples, and distinguish core features from peripheral attributes to improve definitional adequacy.

Relationship to related concepts: underdefinition is distinct from underdetermination of theory by evidence, which concerns how data may be compatible with multiple theories rather than how a term is defined. See also overdefinition, vagueness, and definitional adequacy.