Acanthosis nigricans is a brown to black, poorly defined, velvety hyperpigmentation of the skin, usually present in the posterior and lateral folds of the neck, the axilla, groin, umbilicus, and other areas.
The most common cause would be insulin resistance. Other causes are familial, obesity, drug-induced, malignant and idiopathic.
People with acanthosis nigricans should be screened for diabetes.