Final answer:
The white crescent-shaped area over the nail matrix at the base of the nail is called the lunula, a visible dense layer of epithelium indicative of active nail growth.
Step-by-step explanation:
The white crescent area located over the nail matrix at the base of the nail is called the lunula. The lunula, which appears as a whitish crescent-shaped region, represents a visible part of the nail matrix. It is a thick layer of epithelium that lies underneath the nail, and it is the part where the nail body forms, indicating the area of active cell growth. As the nail matrix cells underneath the lunula proliferate and differentiate, they generate the nail plate that continues to grow across the nail bed. The lunula's pronounced white appearance is due to the dense tissue that obscures the underlying blood vessels which otherwise make the rest of the nail bed appear pink.