Full Idea
An idea is an immediate inner object, which expresses the nature or qualities of things, ..but since it is the object of thought it can exist before and after the thoughts.
Gist of Idea
An idea is an independent inner object, which expresses the qualities of things
Source
Gottfried Leibniz (New Essays on Human Understanding [1704], 2.01)
Book Reference
Leibniz,Gottfried: 'New Essays on Human Understanding', ed/tr. Remnant/Bennett [CUP 1996], p.109
A Reaction
This sounds something like Frege's 'third realm' between mind and world (Idea 7740). Notice that Leibniz is also using the word 'object' in this context. Leibniz doesn't make the mistake of confusing concepts and images, as many did.
Related Idea
Idea 7740 There exists a realm, beyond objects and ideas, of non-spatio-temporal thoughts [Frege, by Weiner]