A person's niece's son would be called a grandnephew, and the person would be a great aunt or uncle. In general, as new generations are added, aunts, uncles and grandparents keep their titles and just add on a "great" for each new generation.
In some areas, people use the prefix "grand" instead of "great" to show that a new generation has been added, and the prefixes are applied to both the aunt, uncle or grandparent and the niece, nephew or grandchild. For instance, a great grandmother's grandchildren's grandchildren would be called her great grandchildren. The genealogy nomenclature for cousins is slightly different, with successive generations being numbered; a person's cousin's child would be called the person's second cousin.