Full Idea
The real definition displays the possibility of the definiendum, and the nominal definition does not.
Clarification
The 'definiendum' is the thing to be defined
Gist of Idea
Real definitions, unlike nominal definitions, display possibilities
Source
Gottfried Leibniz (New Essays on Human Understanding [1704], 3.03)
Book Reference
Leibniz,Gottfried: 'New Essays on Human Understanding', ed/tr. Remnant/Bennett [CUP 1996], p.295
A Reaction
This, I take it, is because the real definition indicates the actual powers of the thing, and not just the superficial characteristics. Is knowledge of powers identical with knowledge of possibilities?