Your father's niece is your first cousin. Her son is your first cousin, once removed.
Your niece and your son are second cousins. Your niece's child and your son are second cousins once removed.
It does not appear that Thomas Jefferson had any nieces or nephews. He had a daughter, a son in law and two grandsons. He also had an aunt and a first cousin.
The children born to your siblings are your nieces and nephews.
Julius Caesar's dad's name was also Gaius Julius Caesar. The Romans named their eldest son after the father.
Nancy
You call them your nieces, or by their names.
You would refer to them as your nieces, thought they are formally your wife's nieces. And they would normally call you uncle.
Your granddaughters and son have the relationship of nieces and uncle. Your granddaughters being his nieces and your son being their uncle.
Second Cousin
Some would call your nephew's children your grand nephews and grand nieces, others would call then your great nephews and great nieces.
Your niece's son is your grand nephew, and you are his great aunt.
Nieces.
Dads typically call their sons by their first names or nicknames.
Your niece and your son are second cousins. Your niece's child and your son are second cousins once removed.
If it is a boy then it is your nephew. If it is a girl then it is your niece.
Some common terms of endearment or nicknames that dads may call their sons include "buddy," "champ," "sport," "son," "pal," "buddy," "little man," "kiddo," "chief," and "junior."
u call your dads brother uncle in English