A carcinoma is any malignant cancer that arises from epithelial cells. Carcinomas invade surrounding tissues and organs and may metastasize, or spread, to lymph nodes and other sites.

”Carcinoma in situ” (CIS) is a pre-malignant condition, in which some cytological signs of malignancy are present, but there is no histological evidence of invasion through the epithelial basement membrane.

Carcinoma, like all neoplasia, is classified by its histopathological appearance. Adenocarcinoma and squamous cell carcinoma, two common descriptive terms for tumors, reflect the fact that these cells may have glandular or squamous cell appearances respectively. Severely anaplastic tumors might be so undifferentiated that they do not have a distinct histological appearance (undifferentiated carcinoma).

Sometimes a tumor is referred to by the presumptive organ of the primary (eg carcinoma of the prostate) or the putative cell of origin (hepatocellular carcinoma, renal cell carcinoma).

Metastatic carcinoma can be diagnosed through biopsy, including fine-needle aspiration, core biopsy, or subtotal removal of single node.