DEFINITION. — As a logical function, definition (definire: to delimit, to set boundaries) is the act by which thought clarifies and determines the content of a concept, resolving it into its essential constituent elements; objectively, it is this conceptual determination in which what the concept is is expressed, and therefore also what the thing signified by it is. In fact, the concepts by which human thought expresses reality generally present themselves as indistinct notions requiring further work of analysis and discernment in order to rise to the level of clear scientific knowledge. Thus definition comes to stand, in relation to the notion defined, as a distinct and scientific concept in contrast to a confused and initial concept. In logic, various kinds of definition are commonly distinguished, although they are not uniformly accepted by everyone, since the terminology fluctuates and the metaphysical and epistemological presuppositions involved differ. Traditional logic distinguishes verbal definition, which is simply an explanation of a word; nominal definition, which is an initial clarification of a thing derived from the etymological meaning of the word; and real definition, which expresses what a thing is through the determination of the objective content of the concept. Real definition is further subdivided, in a descending gradation of ontological perfection and penetration, into essential (the only definition properly so called), causal, genetic, and descriptive definition.
A terminology deriving from Kant divides definition into synthetic, which is achieved through the constructive movement of forming a new total concept from elements or parts, and analytic, which resolves a given concept into its partial notes.
The first to highlight the importance of definition was Socrates, who held the purpose of science to consist in determining the exact value of concepts, what each thing is. This notion of definition as the determination of the essence of a thing is found in Plato (Theaet., 209 A), and especially in Aristotle, who develops a systematic theory of definition, distinguishing between real and nominal definition, establishing the fundamental laws of essential definition and the corresponding methods of inquiry (cf. Anal. post., II, 3, 7, 13; Top., I, 5, 8; VI, 4; De anim., II, 2, 1). Scholasticism retained and developed the Aristotelian theory; St. Thomas: «definito indicat rei quidditatem et essentiam» (Sum. Theol., 2ᵃ-2ᵇᶜ, q. 4, a. 1, in c.). In modern logic, amid the greatest variety of terminology and evaluations of the scientific value of definition, the tendency to deny the distinction between nominal and real definition has become more pronounced. For Pascal, Reid, and Stuart Mill, definitions are nothing other than explanations of words. Others regard definition as a purely clarifying activity concerning concepts, with merely methodological value. It is undoubtedly true that every definition is first and foremost a determination of a concept by means of other concepts and, necessarily, also an explanation of the word by which that concept is expressed; but this does not prevent definition from having the scope of clarifying and determining reality itself. Indeed, once the objectivity of our concepts is admitted, to the extent of that objectivity every objective determination of the concept is also a noetic determination of reality in its essential contents, which, more or less distinctly, are given in the concept. Even so, one can neither expect nor require definition to undertake the task of exhausting the logical expression of the real.
Logic establishes various laws or norms for a good definition, the principal one of which requires that the definition be adequate, that is, such as applies to all and only the subjects included under the concept defined; and consequently, with regard to essential definition, that it express the proximate genus and the specific difference (v. PREDICABILI).