http://www.mayoclinic.org/diseases-conditions/actinic-keratosis/basics/definition/con-20030382

A precancerous lesion of the skin composed of atypical keratinocytes. It is characterized by the presence of thick, scaly patches of skin. Several histologic variants have been described, including atrophic, acantholytic, and hyperkeratotic variants.

An actinic keratosis (ak-TIN-ik ker-uh-TOE-sis) is a rough, scaly patch on your skin that develops from years of exposure to the sun. It's most commonly found on your face, lips, ears, back of your hands, forearms, scalp or neck. Also known as solar keratosis, an actinic keratosis enlarges slowly and usually causes no signs or symptoms other than a patch or small spot on your skin. These lesions take years to develop, usually first appearing in older adults. A small percentage of actinic keratosis lesions can eventually become skin cancer. You can reduce your risk of actinic keratosis by minimizing your sun exposure and protecting your skin from ultraviolet (UV) rays.