Full Idea
By 'idea', I mean the mental conception which is formed by the mind as a thinking thing (this is not a passive perception with regard to the object, but expresses an activity of the mind).
Gist of Idea
An 'idea' is a mental conception which is actively formed by the mind in thinking
Source
Baruch de Spinoza (The Ethics [1675], II Def 3)
Book Reference
Spinoza,Benedict de: 'Ethics, Improvement of Understanding, Letters', ed/tr. Elwes,R [Dover 1955], p.82
A Reaction
This is interesting as a seventeenth century attempt to grapple with the nature of thought. Spinoza sees it as of the essence of mind, since it is what the mind contributes, rather than what happens to the mind when it experiences.