Pathology dictionary -
Undifferentiated is a word used to describe cancer cells that do not look anything like the normal cells usually found in that location of the body.
Undifferentiated is on one end of a scale that pathologists call differentiation. This scale is used to compare cancer cells to normal cells. The scale ranges from well differentiated to undifferentiated. Well differentiated cells look the most like normal cells while undifferentiated cells look the least (or nothing) like normal cells.
Your pathologist may order additional tests, such as immunohistochemistry, to better understand an undifferentiated cancer and to try to determine where the cancer started.
Patients with undifferentiated cancers tend to have worse prognosis than patients with more differentiated cancers.