What does the term 'porosity' refer to in hair?

Porosity refers to the hair's ability to absorb moisture, which is a critical factor in hair care and styling. Understanding porosity helps cosmetologists determine how well hair can take in and hold moisture, as well as how it reacts to various hair products and treatments. High porosity hair tends to absorb moisture quickly but can also lose it just as fast, making it important to use products that provide adequate hydration. Conversely, low porosity hair may struggle to absorb moisture, requiring specific techniques or lighter products to aid in moisture penetration.

While the other choices mention heat retention, density, and color characteristics, they do not pertain to the fundamental property of how hair interacts with moisture, which is essential for maintaining healthy hair.