DEFINITION. - As a logical function, definition (definire: to delimit, to set the 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 is expressed what the concept is, and therefore also what the thing signified by it is. In fact, the concepts with which human thought expresses reality usually present themselves for the most part as indistinct condensations that require a further work of analysis and discernment in order to ascend 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 comparison with a confused and initial concept. In logic it is customary to distinguish various species of definition, which, however, are not accepted by all indiscriminately, the terminology being fluid and the metaphysical and epistemological presuppositions involved being diverse. Traditional logic distinguishes verbal definition, which is a simple explanation of a word; nominal definition, which is an initial clarification of a thing, drawn from the etymological meaning of the word; real definition, which is the expression of what a thing is, through the determination of the objective content of the concept. Real definition is further subdivided, with a descending gradation of perfection and ontological penetration, into essential (the only definition properly so called), causal, genetic and descriptive.
A terminology deriving from Kant divides definition into synthetic, which is brought about by a constructive movement of a new total concept from elements or parts, and analytic, which resolves a given concept into its partial notes.
The first to bring to light the importance of definition was Socrates, who made the aim of science 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 particularly in Aristotle, who develops a systematic theory of definition, distinguishing between real and nominal definition, establishing the fundamental laws of essential definition and the related methods of inquiry (cf. Anal. post., II, 3, 7, 13; Top., I, 5, 8; VI, 4; De anim., II, 2, 1). Scholasticism held to the Aristotelian theory, developing it; 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 of appraisal regarding the scientific value of definition, there is an accentuated tendency to deny the distinction between nominal and real definition. For Pascal, Reid, Stuart Mill, definitions are nothing other than explanations of words. Others regard definition as a pure clarification of concepts, with a merely methodological value. It is undoubtedly true that every definition is first of all 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 being able to have a bearing on the clarification and determination of reality itself. In fact, once the objectivity of our concepts is granted, to that extent every objective determination of the concept is also a noetic determination of reality in its essential contents, such as, more or less distinctly, are given in the concept. Even so, however, one cannot and must not expect from definition a task of exhaustively expressing the real in logical terms.
Logic establishes various laws or norms for a good definition, among which the principal one requires that the definition be adequate, that is, such that it applies to all and only the subjects included under the concept defined; and consequently, as regards essential definition, that this be an expression of the proximate genus and the specific difference (v. PREDICABILI).