<unk>  is a medical term for white discoloration appearing on nails.  It is derived from the Greek words Leuko white and Onyx nail.  The most common cause is injury to the base of the nail (the matrix) where the nail is formed.
Leukonychia
Melanonychia
Paronychia
Platonychia
Anonychia
Koilonychia
Aspermia
Pachyonychia congenita