What is a nevus? A proposed definition of a common medical term.

From the study of standard textbooks of dermatology, no comprehensive understanding of the word nevus can be derived. The purpose of this article is to formulate a workable definition. As a first step, various definitions proposed by previous authors are reviewed. Their inconsistency appears to be mainly due to the fact that the term nevus is used to denote both neoplastic and nonneoplastic, congenital and acquired as well as hereditary and nonhereditary skin lesions. Taking this difficulty into account, the author suggests, as a unifying concept, the new category of genetic mosaicism. The following definition is proposed: 'Nevi are visible, circumscribed, long-lasting lesions of the skin or the neighboring mucosa, reflecting genetic mosaicism. With the exception of melanocytic nevi, they do not show neoplastic growth. They never show malignant neoplasia.'--For several types of nevi the concept of genetic mosaicism has already been confirmed at the cellular level. Future cytogenetic and molecular studies should show whether the proposed definition is valid for all other types of nevi and thus generally acceptable.