Perspective
Perspective is one of the first things that you learn when you begin to discover more about drawing and art. I think it might have been when I was in elementary school in grade 3 that I first learned about the idea of a vanishing point on the horizon which serves as a reference point for the angles in a drawing, giving the image a sense of depth and the illusion of realism. As drawing technique becomes more advanced and complex, artists may further enhance their work with two-point and three-point perspective drawing.
As you learn more about the concept, you discover how revolutionary the idea was in the development of art. Now, we take for granted the craft of image making, as cameras have become ubiquitous. At the time of the Renaissance, the discovery, or rediscovery, of perspective was a very significant advancement in the science and mathematics of optics and its application to the art and craft of painting.