Overview
A definition is an explicit statement that specifies the meaning, nature, or boundaries of something. It typically answers the question "what is it?" by identifying essential characteristics of a term. In common usage a definition explains a word or a phrase, and is closely related to a description or explanation used to clarify use and context.
Characteristics of useful definitions
Good definitions aim for clarity, minimal ambiguity, and appropriateness for their purpose. Typical criteria include non-circularity, sufficient precision to distinguish the defined item from others, and economy of wording. Formal definitions often use a genus-and-differentia structure (a general category plus distinguishing features), while informal definitions may rely on examples or ostension.
History and development
Definitional practice has roots in ancient philosophy and rhetoric, where thinkers analyzed how words relate to things and concepts. Over time traditions diverged: lexicographers developed descriptive entries for living languages, logicians and philosophers pursued exact conceptual definitions, and scientists and jurists created operational or legal definitions to fix criteria for measurement, classification, or regulation.
Definitions in formal disciplines
In disciplines such as mathematics, computer science, and logic, definitions are precise stipulations that introduce symbols and concepts so they can be manipulated within proofs and arguments. A mathematical definition is chosen for exactness and usefulness within a theory, and it is often referenced in a proof or derivation where formal consequences are established.
Uses and examples
Definitions perform different roles: lexical definitions summarize common usage; stipulative definitions assign a meaning for a particular discussion; precising definitions reduce vagueness in legal or technical contexts; and theoretical definitions propose a framework within a scientific theory. A classic example in teaching is defining "bachelor" as an unmarried adult man — simple but open to scope questions in special contexts.
Distinctions and practical guidance
It is important to distinguish a definition from a mere description or an explanation of causes. When writing definitions avoid circular wording, include necessary and sufficient traits where possible, and tailor the formality to the audience. Recognizing the type of definition needed—lexical, stipulative, operational, or theoretical—helps ensure it serves communication, analysis, or measurement effectively.