A "perfect" version of something in the world. Plato believed that every object had a perfect form. He used the argument that we all have a general image of an object in our mind that we use as a reference. We have an image for a horse, a triangle, a house, etc. Since we have these images, and we use them regularly, it followed that there must somewhere BE these perfect forms. A perfect horse, a perfect triangle, a perfect house. These are called Platonic Forms.