The term idea has had a number of special uses in the history of philosophy which do not correspond to its modern meaning.
In ancient philosophy, Plato used the word 'eidos', often translated as 'idea', to refer to his notion of a Platonic Form, an immutable, non-physical entity corresponding to what we would call a property, like Goodness or Beauty. Plato believed that the only true knowledge was knowledge of these unchanging ideas.
In early modern philosophy, by contrast, 'idea' was a broad term usually taken to cover 'ideas of sensation' (perceptual experiences and other sensations). The origin for this usage was Locke's Essay Concerning Human Understanding, in which he defines it as "that term which, I think, serves best to stand for whatsoever is the object of the understanding when a man thinks, I have used it to express whatever is meant by phantasm, notion, species, or whatever it is which the mind can be employed about in thinking; and I could not avoid frequently using it." David Hume would later restrict this definition to cover mental reconstructions of perceptual experiences, using the term 'impressions' to refer to perceptual experiences themselves.