Full Idea
Of the essence of a particular thing is what pertains to it necessarily and perpetually; of the concept of an individual thing on the other hand is what pertains to it contingently or per accidens.
Gist of Idea
The essence is the necessary properties, and the concept includes what is contingent
Source
Gottfried Leibniz (Human Freedom and Divine choice [1690], Grua 383), quoted by Cover,J/O'Leary-Hawthorne,J - Substance and Individuation in Leibniz 3.3.1
Book Reference
Cover,J/O'Leary-Hawthorne,J: 'Substance and Individuation in Leibniz' [CUP 1999], p.127
A Reaction
This arbitrates on the apparent conflict between his remarks in Idea 13077 and Idea 10382. There seems to be a distinction between the 'concept' of a thing, and the 'complete concept', the latter including the contingent properties.
Related Ideas
Idea 13077 Basic predicates give the complete concept, which then predicts all of the actions [Leibniz]
Idea 13082 The complete concept of an individual includes contingent properties, as well as necessary ones [Leibniz]