(1) [TA], The corneal layer of epidermis overlapping and in direct contact with the nail root proximally or the sides of the nail plate laterally, forming the undersurface of the nail wall or nail folds. SYN: hidden nail skin, perionychium. (2) The thin, condensed, eleidin-rich layer of epidermis that precedes and initially covers the nail plate in the embryo. It normally degenerates by the eighth month except at the nail base where it remains as the cuticle of the nail. (3) The thin skin adherent to the nail at its proximal portion. SYN: epionychium. [G. epi, upon, + onyx (onych-), nail]