Acanthosis nigricans is a brown to black, poorly defined, velvety hyperpigmentation of the skin. It is usually found in body folds, such as the posterior and lateral folds of the neck, the armpits, groin, navel, forehead, and other areas.
- Benign This may include obesity-related, hereditary, and endocrine forms of acanthosis nigricans.
- Malignant. This may include forms that are associated with tumour products and insulin-like activity, or tumour necrosis factor 
An alternate classification system still used to describe acanthosis nigricans was proposed in 1994 by dermatologist Schwartz. This classification system delineates acanthosis nigricans syndromes according to their associated syndromes, including benign and malignant forms, forms associated with obesity and drugs, acral acanthosis nigricans, unilateral acanthosis nigricans, and mixed and syndromic forms
It typically occurs in individuals younger than age 40, may be genetically inherited, and is associated with obesity or endocrinopathies, such as hypothyroidism, acromegaly, polycystic ovary disease, insulin-resistant diabetes, or Cushing's disease.
- states with insulin resistance, such as diabetes mellitus
- excess circulating androgens, particularly Cushing's disease, acromegaly, polycystic ovarian disease
- Addison's disease and hypothyroidism
- Rare diseases, including pinealoma, leprechaunism, lipotrophic diabetes, pineal hyperplasia syndrome, pituitary basophilism, ovarian hyperthecosis, stromal luteoma, ovarian dermoid cysts, Prader-Willi syndrome, and Alstrom syndrome.
Acanthosis nigricans associated with endocrine dysfunction is more insidious in its onset, is less widespread, and the patients are often concurrently obese.:676 This is also known as "Acanthosis nigricans type III",:506–7 and insulin resistance syndromes may be divided into Type A (HAIR-AN) and type B syndromes.:978
The majority of cases of acanthosis nigricans are associated with obesity and otherwise idiopathic. This is likely because of insulin resistance, and more likely to occur in darker-skinned persons.:968 This is also known as "Pseudo-Acanthosis Nigricans", or "Type 3 Acanthosis Nigricans".:86