The measurement that describes the number of individual hair strands on one square inch of the scalp is:

The measurement that describes the number of individual hair strands on one square inch of the scalp is density. Density refers to how closely packed the hair follicles are within a given area, which directly correlates to the overall appearance of the hair. High density indicates a greater number of hair strands, contributing to a fuller look, while low density suggests fewer strands, which may result in a sparser appearance.

Length refers to how long the hair strands are and does not relate to the quantity of hair in a given area. Thickness pertains to the diameter of each individual hair strand, affecting the texture but not the number of strands present. Porosity describes the hair's ability to absorb moisture and allows stylists to evaluate how well hair can retain hydration, which is unrelated to strand count. Understanding density is vital for estheticians when considering haircuts, styles, and treatments best suited for different hair types and conditions.