Acanthosis nigricans: A skin condition characterized by dark thickened velvety patches, especially in the folds of skin in the axilla (armpit), groin and back of the neck. The condition is complex. It can occur with endocrine diseases such as Cushing disease, tumors of the pituitary, and diabetes mellitus. It is common in people who have insulin resistance — whose body is not responding correctly to the insulin that they make in their pancreas. Acanthosis nigricans also occurs with underlying malignancies (especially carcinomas of the vicera), administration of certain drugs, and as a genetic disorder inherited in an autosomal dominant manner.