The Merriam-Webster definition of “shape” that we are concerned with as photographic artists is:
1 a : the visible makeup characteristic of a particular item or kind of item
1 b (1) : spatial form or contour
1 b (2) : a standard or universally recognized spatial form
Shapes are two-dimensional. They can be measured by overall height and width. Shapes can be the outline of an object—familiar or unfamiliar.