Adenocarcinoma is a type of cancerous tumor that can occur in several parts of the body.
It is defined as neoplasia of epithelial tissue that has glandular origin, glandular characteristics, or both.
Adenocarcinomas are part of the larger grouping of carcinomas.
In the narrowest sense, the glandular origin or traits are exocrine; endocrine gland tumors, such as an insulinoma, or a pheochromocytoma, are typically not referred to as adenocarcinomas but rather are often called neuroendocrine tumors.
Epithelial tissue sometimes includes, but is not limited to, the surface layer of skin, glands, and a variety of other tissue that lines the cavities and organs of the body. Epithelial tissue can be derived embryologically from any of the germ layers (ectoderm, endoderm, or mesoderm). To be classified as adenocarcinoma, the cells do not necessarily need to be part of a gland, as long as they have secretory properties.
Adenocarcinoma is the malignant counterpart to adenoma, which is the benign form of such tumors.

Well-differentiated adenocarcinomas tend to resemble the glandular tissue that they are derived from, while poorly differentiated adenocarcinomas may not.
Adenocarcinomas can arise in many tissues of the body owing to the ubiquitous nature of glands within the body, and, more fundamentally, to the potency of epithelial cells.

Wikipedia, the free encyclopedia. Internet. Accessed on January 18, 2016.