Final answer:
The eponychium, also known as the cuticle, is the extension at the base of the nail body that partially overlaps the lunula and protects the nail bed.
Step-by-step explanation:
The extension of the cuticle at the base of the nail body which partly overlaps the lunula is called the eponychium, also commonly referred to as the cuticle. The eponychium is a fold of epithelium that extends over the nail bed, meeting at the proximal end of the nail body and serving as a protective barrier. It is one of several important structures in the anatomy of the nail, which includes the nail body, nail fold, nail root, lunula, and nail matrix - the latter of which is responsible for nail growth as it contains stem cells that divide to form keratinocytes. This intricate system is essential for protecting the tips of fingers and toes and for fine motor functions such as picking up small objects.