Ichthyosis vulgaris: A genetic skin disease characterized by scaly areas of skin that usually appear in childhood. The palms and soles are often affected. Areas that tend to be spared include the armpits, the insides of the elbows, and the skin behind the knee. Many people with ichthyosis vulgaris also have asthma, eczema, or hay fever. The gene responsible for this disease is located on chromosome 1. Also known as ichthyosis simplex.