A definition is the condensation of a vast body of observations - and stands or falls with the truth or falsehood of these observations.
The truth or falsehood of all man's conclusions, inferences, thought and knowledge rests on the truth or falsehood of his definitions.
A contextual definition can be formulated only after one has fully considered all the known facts pertaining to the units in question.
Although the definition explicitly mentions only the essential characteristic(s), it implies and condenses all of this knowledge.
On a rational view of definitions, a definition organizes and condenses - and thus helps one to retain - a wealth of knowledge about the characteristics of a concept's units. On the nominalist view, it is precisely this knowledge that is discarded when one defines a concept: as soon as the defining characteristic is chosen, all the other characteristics of the units are banished from the concept, which shrivels to mean merely the definition.
A definition cannot list all the characteristics of the units; such a catalog would be too large to retain. Instead, a definition identifies a concept's units by specifying their essential characteristics. The essential characteristic(s) is the fundamental characteristic(s) which makes the units the kind of existents they are and differentiates them from all other known existents.