Calvin in 2-D Form refers to one of Calvin's alter egos. Ir appeared in a single Sunday strip. It is a flat version of Calvin.
The alter ego first appeared as a two-dimensional version of Calvin with height and width but no depth (hence the "2-D" epithet), who interacted with Calvin's regular surroundings. He claimed to be "thinner than a sheet of paper...Having width but no thickness..." As a result, when standing up straight, he becomes a "nearly invisible vertical line", although his shadow can give him away.
Calvin's 2-D form must only have been temporary; Calvin told his father that he had been two-dimensionnal "all morning", implying that at the time of telling his father he had returned to his normal form. Calvin offered this explanation for his absence during the morning; as such, Calvin In 2-D Form was here analogous to Calvin's absence.