What characteristic defines the hair's wave pattern?

The characteristic that defines the hair's wave pattern is shape. The wave pattern of hair refers to the natural design and formation of the strands, which influences whether hair is straight, wavy, curly, or coily. This shape is determined by the structure of the hair follicle and affects how the hair lays and behaves when styled.

The other options, although they are important aspects of hair, do not specifically define the wave pattern. For instance, texture refers to the feel and thickness of the hair strands, color pertains to the pigment present in the hair, and volume relates to the fullness or body of the hair. While all these attributes contribute to the overall appearance of hair, the shape specifically identifies the wave pattern, making it the correct choice.