acanthosis nigricans

noun

acanthosis ni·​gri·​cans -ˈnig-rə-ˌkanz  -ˈnī-grə- 

: a skin condition that is marked by dark discoloration and velvety thickening of the skin especially of body folds or creases (as of the armpit, groin, or neck) and that is often associated with obesity, insulin resistance, hormonal irregularity, or medication use but may rarely occur as a sign of cancer (as of the stomach or liver)