Onychoatrophy
Encyclopedia
Onychoatrophy is a faulty underdevelopment of the nail
Nail (anatomy)
A nail is a horn-like envelope covering the dorsal aspect of the terminal phalanges of fingers and toes in humans, most non-human primates, and a few other mammals. Nails are similar to claws, which are found on numerous other animals....

 that may be congenital or acquired, in which the nail is thinned and smaller.
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