Definition (Lat. definitio, from de-finire, to set limits to, describe), a logical term used popularly for the process of explaining, or giving the meaning of, a word, and also in the concrete for the proposition or statement in which that explanation is expressed. In logic, definition consists in determining the qualities which belong to given concepts or universals; it is not concerned with individuals, which are marked by an infinity of peculiarities, any one or all of which might be predicated of another individual. Individuals can be defined only in so far as they belong to a single kind. According to Aristotle, definition is the statement of the essence of a concept (ὁρισμὸς μὲν γὰν τοῦ τί ἐστι καὶ οὐσίας, Posterior Analytics, B iii. 90 b 30); that is, it consists of the genus and the differentia. In other words, “man” is defined as “animal plus rationality,” or “rational animal,”1 i.e. the concept is (1) referred to the next higher genus, and (2) distinguished from other modes in which that genus exists, i.e. from other species. It is sometimes argued that, there being no definition of individuals as such, definition is of names (see J. S. Mill, Logic, i. viii. 5), not of things; it is generally, however, maintained that definition is of things, regarded as, or in so far as they are, of a kind. Definition of words can be nothing more than the explanation of terms such as is given in a dictionary.
The following rules are generally given as governing accurate definition. (1) The definition must be equivalent or commensurate with that which is defined; it must be applicable to all the individuals included in the concept and to nothing else. Every man, and nothing else, is a rational animal. “Man is mortal” is not a definition, for mortality is predicable of irrational animals. (2) The definition must state the essential attributes; a concept cannot be defined by its accidental attributes; those attributes must be given which are essential and primary. (3) The definition must be per genus et differentiam (or differentias), as we have already seen. These are the important rules. Three minor rules are: (4) The definition must not contain the name of the concept to be defined; if it does, no information is given. Such a proposition as “an archdeacon is one who performs archidiaconal functions” is not a definition. Concepts cannot be defined by their correlatives. Such a definition is known as a circulus in definiendo. (5) Obscure and figurative language must be avoided, and (6) Definitions must not be in the negative when they can be in the affirmative.
1 “Rational animal” is thus the predicate of the statement constituting the definition. Sometimes the word “definition” is used to signify merely the predicate.
