A definition first
A thought or suggestion as to a possible course of action: "the idea of linking pay to performance".
A concept or mental impression.
And from Wikipedia
An idea is a concept or mental impression.
[1] Ideas are often construed as representational images; i.e., images of some object. In other contexts, ideas are taken to be concepts, although abstract concepts do not necessarily appear as images.
[2] Many philosophers consider ideas to be a fundamental ontological category of being. The capacity to create and understand the meaning of ideas is considered to be an essential and defining feature of human beings. In a popular sense, an idea arises in a reflex, spontaneous manner, even without thinking or serious reflection, for example, when we talk about the idea of a person or a place.
In my Teach yourself History of philosophy book (by John Lewis) I learn that Plato held the view that all general ideas, principles and ideals have an existence independent of the perceiving mind. - Lewis goes on to say (paraphrased view of Plato on this) we do not make but find them.
In marked contrast Aristotle (Plato's teacher) believed in General Ideas which he called forms or essences.
