Patients with cancer of unknown primary origin have metastatic malignant disease without an identifiable primary site.
For patients with this condition the type of tumor, the extent of its spread and the outcome of treatment vary widely.

Most patients have malignancy that appears to derive from epithelial cells, that is, carcinoma of unknown primary origin (CUP).
Patients with tumors of non-epithelial lineage (melanoma, sarcoma, lymphoma, germ cell) form a distinct minority because their tumors can often be managed satisfactorily even without an identifiable primary site.

A patient who presents with metastatic malignancy (in the form of tumor masses or effusions) identified on clinical examination or by imaging, without an obvious primary site, is regarded as having malignancy of undefined primary origin (MUO).
Provisional carcinoma of unknown primary origin (provisional CUP) is used to refer to patients with metastatic malignancy of proven epithelial, neuro-endocrine, or undifferentiated lineage after initial, but not exhaustive, investigations. Although a primary site will be found in most of these patients, or a non-epithelial malignancy diagnosed, in some patients a primary site will not be found and a diagnosis of 'provisional CUP' will change to a diagnosis of 'confirmed CUP' after the results of all tests are complete.

See reference for details.

National Institute for Health and Care Excellence (NICE). Metastatic malignant disease of unknown primary origin in adults: diagnosis and management. Guideline July 2010. Internet. Accessed on May 27, 2016.