A definition is a meaning that uses a word or a phrase to mark off the conceptual limits of what a thing is.
A definition of a name (definitio nominis) sets a clear value for a word or phrase so that the parties involved will definitely know what that word/phrase means when they talk about the word/phrase. It is a common establishment not only in language, but also in logic and mathematics. For example, I call soul that which is the principle of thought within us.
A definition of a thing (definitio rei) is the maintenance that other ideas are contained in a certain term (assuming common usage) referring to a thing. For example, Man is a rational animal.
A definition of a name is arbitrary; it can choose one narrower definition of a word extracted from a broader commonly used definition, assign a word an analogous meaning, give a word an entirely new meaning, or even apply itself to an entirely new word. The ideas employed in a definition of a thing on the other hand are not arbitrary but must apply themselves to comprehending their object and to the extent that they fail to do so, they must be redetermined.
Academic and Technical Definitions
Many words have multiple definitions of similar connotation, such as line. A line can refer to a finite group of people in a row or a stripe of paint along the highway, but in geometry, it refers strictly to an infinite mass of one dimension which can exist in other dimensions. The definition in geometry would be its academic definition.
- Arnauld, Antoine (1662). Logic or the Art of Thinking: Being The Port Royal Logic. Ed. and trans. T. S. Baynes. London: W. Blackwood, 1850 (La logique ou l'art de penser; contenant outre les règles communes, plusieurs observations nouvelles, propres à former le jugement, Paris), pp. 78-9.
- Merriam-Webster's Third International Dictionary. 1960.
- The American College Dictionary. Random House. New York, 1956
- "1.2: Points, Lines, and Planes." Beauty, Rigor, Surprise.