Squamous cell carcinoma in situ: An early stage of skin cancer that develops from squamous cells (the flat, scale-like cells in the outer layer of the skin). The hallmark is a persistent, progressive, slightly raised, red, scaly, or crusted plaque that may occur anywhere on the skin surface or on mucosal surfaces, such as in the mouth. Under a microscope, atypical squamous cells are seen to have proliferated through the whole thickness of the epidermis (the outer layer of the skin) but not beyond. Squamous cell carcinoma in situ is commonly caused by sun exposure, but can be from prolonged exposure to arsenic. Also known as Bowen disease.