Final answer:
An outline is the type of line that creates a boundary around a flat shape, serving as the defining edge of a shape in a piece of artwork.
Step-by-step explanation:
The type of line that acts as a boundary line, creating an edge around a flat shape, is known as an outline. Outlines are the simplest form of line in artistic composition, setting the perimeter around the shape and distinguishing it from the background or other forms. They can be seen as the path that marks the outer edge of a shape and are essential for defining shapes in a two-dimensional space. Unlike contour lines which show variations in surface features and can give the impression of three-dimensionality, outlines strictly define the shape's external boundary.
Other types of lines include expressive lines, which are curved and add an organic character to a piece of art, and implied lines, which suggest a continuation of a line beyond the actual end point. Contour lines, on the other hand, differ from outlines by mapping variations within a shape, such as in a topographical map that shows elevation changes. Understanding the different line qualities can greatly aid in appreciating and creating art.