When referring to something that lacks depth, the word "two-dimensional" usually springs to mind. However, there are several synonyms you can use in order to vary your vocabulary and avoid repetition. The phrase "flat" can describe an object or image that appears to exist on a two-dimensional plane, while "plane" itself denotes the same idea of a level surface. "Shallow" can also be used as a synonym, as it connotes a lack of depth and dimensionality. "Static" can be employed in the context of media or artwork, as it suggests that the subject remains still and does not have a sense of depth or movement. Finally, "lacking depth" or "lacking dimensionality" can be used to describe something as two-dimensional without relying on the word itself.