What is the term for an abnormal growth of the skin?

The term for an abnormal growth of the skin is hypertrophy. Hypertrophy specifically refers to an increase in the size or mass of cells within a tissue or organ, leading to an enlargement. In the context of skin, this can manifest as thickened areas or lumps that are abnormal compared to surrounding skin.

Dermatosis refers more broadly to any skin disease or disorder, but it does not specifically denote growth. Atrophy describes a reduction in size or wasting away of an organ or tissue, which is the opposite of what is being described in the question. Hyperplasia, while it does involve an increase in the number of cells, is more commonly used in contexts relating to tissues and organs in general rather than specifically regarding the skin, and it does not imply the same level of abnormality in growth as hypertrophy does in the context of the skin. Thus, hypertrophy is the most accurate term for describing abnormal skin growth.