Why is a single layer of atoms referred to as "2d"? I commonly hear a single layer of atoms referred to as being a "2-dimensional structure", when it so clearly is not 2-dimensional. Why do people make that obvious mistake? The same exact folks will also tell you that spacetime is a continuum, but by that logic, we should be able to split that "2-dimensional structure" up into infinitely smaller 3d volumes, correct? How is this reconciled?