Hyperpigmentation is the process of darkening of a skin or nail area caused by the increased presence of melanin in it.
It can be congenital (like café au lait, nevus of ota and others) or acquired (caused by sun damage, inflammation, or other skin injuries, including those related to acne vulgaris). People with darker (Asian, Mediterranean, or African) skin tones are also more prone to it, especially if they have excessive sun exposure.
Many forms of hyperpigmentation are caused by an excessive production of melanin. It can be diffuse or focal, affecting such areas as the face and the back of the hands.