Definitionally

Definitionally is an English adverb used to indicate that a statement follows from the definition of the term involved, or that a proposition is true by definition. The term signals a logical or conceptual necessity that is a consequence of how something is defined. It is common in formal writing, philosophy, mathematics, and linguistics, though in everyday language speakers often substitute “by definition” or “trivially.”

Example: “Definitionally, a bachelor is an unmarried man.” Another example: “Definitionally, a square is a rectangle with equal sides.” The latter can be somewhat contextual: in mathematics, a square is defined as a special type of rectangle with equal sides; depending on the formal system, one might describe this as true by definition.

In addition to plain usage, phrases such as “definitionally equivalent” or “definitionally true” describe statements that hold purely by the definitions, without empirical evidence. This concept is related to tautology and logical consequence: tautologies are true in all interpretations, while something true by definition depends on the chosen definitions.

Usage notes: the term can appear formal or pedantic, and some audiences prefer “by definition” to avoid ambiguity. In practice, definitionally signals that no further empirical or experiential justification is needed because the truth follows directly from the definitional framework. The word functions as a descriptive label for a certain kind of inference rather than as a mathematical theorem.