Mohs surgery: A type of surgery that is used for the treatment of skin cancer, especially basal cell or squamous cell carcinoma of the skin. Mohs surgery is designed to remove all the cancerous tissue while removing as little of the healthy tissue as possible. This type of surgery is especially helpful when the physician is not sure of the shape and depth of a tumor. In addition, this method is used to remove large tumors, tumors in hard-to-treat places, and cancers that have recurred. Mohs surgery is microscopically controlled. The area of skin is removed under local anesthetic and is then carefully oriented and serially examined under a microscope to ensure that all of the tumor has been removed. If the tumor has not all been removed, the procedure is repeated until the entire tumor is removed.