Full Idea
Definitions to my mind are real, when one knows that the thing defined is possible; otherwise they are only nominal, and one must not rely on them.
Gist of Idea
Definitions can only be real if the item is possible
Source
Gottfried Leibniz (Letters to Antoine Arnauld [1686], 1686.07.4/14 XI)
Book Reference
Leibniz,Gottfried: 'The Leibniz-Arnauld Correspondence', ed/tr. Mason,HT/Parkinson,GHR [Manchester UP 1967], p.72
A Reaction
It is interesting that things do not have to actual to have real definitions. For Leibniz, what is possible will exist in the mind of God. For me what is possible will exist in the potentialities of the powers of what is actual.