A definition is a statement that identifies the nature of the units subsumed
under a concept.
It is often said that definitions state the meaning of words. This is true, but
it is not exact. A word is merely a visual-auditory symbol used to represent a
concept; a word has no meaning other than that of the concept it symbolizes,
and the meaning of a concept consists of its units. It is not words, but
concepts that man defines—by specifying their referents.
The purpose of a definition is to distinguish a concept from all other concepts
and thus to keep its units differentiated from all other existents.
Since the definition of a concept is formulated in terms of other concepts, it
enables man, not only to identify and retain a concept, but also to establish
the relationships, the hierarchy, the integration of all his concepts and
thus the integration of his knowledge. Definitions preserve, not the
chronological order in which a given man may have learned concepts, but the
logical order of their hierarchical interdependence.
With certain significant exceptions, every concept can be defined and
communicated in terms of other concepts. The exceptions are concepts referring
to sensations, and metaphysical axioms.