What describes the thickness of a hair strand?

The correct choice is diameter, as it specifically refers to the measurement across a hair strand, which directly determines its thickness. Diameter helps in understanding the size of the hair shaft itself.

Thickness in terms of hair is an essential characteristic that can influence styling options, product choices, and overall hair health. While density refers to the number of hair strands per square inch on the scalp, and texture often describes the feel or appearance of hair (such as coarse or fine), diameter directly assesses the physical width of each individual strand. Furthermore, volume relates to the overall fullness or body of hair, which is a different concept entirely. Understanding diameter helps stylists cater to individual client needs based on their hair's specific attributes.