Nucleoli are round pieces of genetic material found in a part of the cell called the nucleus. Nucleoli produce the chemicals that cells use to make protein. A cell that needs more protein may increase the number of nucleoli or have very large and easy to see nucleoli.
Pathologists use the word nucleoli when describing multiple round structures. If only one round structure is seen it is called a nucleolus.
Large nucleoli are very common in some types of cancer and pathologists often use nucleoli as proof that a cell may be cancerous. However some normal, healthy cells also have large nucleoli so the context in which the nucleoli are seen is important in making the correct diagnosis.