Form is defined as a 3D object that has thickness or gives the illusion of being 3D through the use of shading and shadows. Forms are distinguished from non-forms by their solid, three-dimensional nature. Shading and shadows are key techniques for representing the three-dimensionality of an object on a two-dimensional surface and making it appear solid rather than flat. The document discusses principles and elements of design, with a focus on defining the concept of form.