Carcinoma is a medical term that refers to an invasive malignant tumor consisting of transformed epithelial cells. Alternatively, it refers to a malignant tumor composed of transformed cells of unknown histogenesis, but which possess specific molecular or histological characteristics that are associated with epithelial cells, such as the production of cytokeratins or intercellular bridges.

Carcinoma in situ (CIS) refers to a small, localized focus of carcinoma that has not yet invaded through the epithelial basement membrane that delimits the carcinoma cells from the adjacent normal tissue. It is important to remember that carcinoma in situ is a pre-invasive cancer, and not a \"pre-malignant\" entity.In most cases, CIS will progress to an invasive carcinoma unless it is completely removed via surgical resection, cryotherapy, or some other treatment modality. In some instances, however, CIS can regress and even disappear, a phenomenon often linked to negation of the original carcinogenic stimulus (i.e. smoking cessation, clearing of HPV virus).