Final answer:
The line that defines the edge of a shape is called the outline. It is a fundamental element in art that delineates the boundary of a shape, essential for defining form and structure in two-dimensional artwork.
Step-by-step explanation:
The line that defines the edge of a shape is called the outline. An outline is the simplest form of line in art that creates a path around the edge of a shape, helping to define its boundary. Whether a shape is geometric (a known shape) or organic (a free form shape), it needs an outline or some kind of boundary, which could be a change in color or an actual line. Without an outline, shapes in two-dimensional art would have no form, and the art piece itself would lack definition and structure.
Contour lines are similar in that they also define shapes. However, they are used to delineate differences in surface features and can give a flat shape a sense of three-dimensionality, such as in contour maps or when used for shading. Lastly, hatching is a technique where closely spaced lines are used for shading, contributing to the illusion of volume and form within two-dimensional artworks.