acanthosis nigricans

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.
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Acanthosis ni·gri·cans n. An eruption of velvety wartlike growths accompanied by hyperpigmentation in the skin of the armpits, neck, anogenital [more]Acanthosis nigricans - Community and Resources

Noun
1. a skin disease characterized by dark wart-like patches in the body folds; can be benign or malignant
(synonym) keratosis nigricans
(hypernym) skin disease, disease of the skin, skin disorder, skin problem, skin condition

skin condition characterized by darkened skin patches; common in people whose body is not responding correctly to the insulin that they make in their pancreas (insulin resistance). This skin condition is also seen in people who have pre-diabetes or type 2 diabetes.