What is defined as a combination of lines that outlines a shape?

The term that best describes a combination of lines that outlines a shape is "form." In the context of hair design, form refers to the three-dimensional quality of the hairstyle that is created by the arrangement of lines and shapes. It encompasses the overall structure and volume of the hairstyle, distinguishing it from the basic two-dimensional aspects.

When thinking about hair design, form plays a crucial role as it affects how a hairstyle appears from different angles and contributes to the overall aesthetic. A well-defined form can enhance a hairstyle's uniqueness and complement the features of the person wearing it.

The other options, while related, do not specifically pertain to the combination of lines outlining a shape in the same way. For instance, silhouette refers to the dark shape and outline of something viewed against a lighter background, which does not directly involve the specifics of line composition in form. Shape focuses more on the two-dimensional representation rather than the three-dimensional aspect inherent in the concept of form. Design, on the other hand, encompasses the overall artistry and arrangement of elements within a hairstyle, but again, it does not specifically pinpoint the lines that outline a shape as "form" does.