The Greek philosopher Plato used the term "idea" to refer to perfect, eternal, and unchanging forms that existed in a realm beyond the physical world. These forms were considered the true reality, and the objects we see in the physical world were mere imperfect copies of these perfect forms.
Over time, the meaning of "idea" shifted to encompass thoughts, concepts, and mental images, although the original sense of "form" and "essence" remains relevant in some contexts.