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.
- 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.