Shape.
An enclosed space defined by a line or by contrast to its surroundings. Shapes are two-dimensional (flat): circle, square, triangle, organic blob, etc. In everyday usage, the word 'shape' is also used to talk about three-dimensional form, often as something of a shorthand for referring to the two-dimensional outline or silhouette of the object. When discussing art, your meaning will be clearer if you reserve using 'shape' to talk about two-dimensional shapes on a plane.