Neoplasia

Neoplasia includes very different diseases, it is difficult to find a definition that describes them all. Definition of the British oncologist RA Willis is widely cited:''neoplasm is an abnormal tissue mass, which exceeds the growth and uncoordinated with the normal tissue, and stick with the same excessive manner after cessation of stimuli that evoke a change.''

This definition has been criticized because some neoplasms, such as nevi, which are not progressive.
Clonality

Neoplastic tumors often contain more than one type of cell, but their initiation and continued growth is usually dependent on a single population of neoplastic cells. These cells are considered to be clonal - that is, they are descended from a single progenitor cell.

Sometimes, neoplastic cells all carry genetic or epigenetic anomaly which becomes evidence for the same clonality. For the lymphoid neoplasms, such as lymphoma and leukemia, clonality proven by rearrangement of their immunoglobulin single gene amplification (for lesions of B cells) or T-cell receptor genes (for lesion T cells). Demonstration of clonality is now considered necessary to identify a neoplastic proliferation of lymphoid cells.

It is tempting to define neoplasms as clonal cellular proliferations but the demonstration of clonality is not always possible. Therefore, clonality is not required in the definition of neoplasia.