A niece is the daughter of your brother or sister. A cousin is the son or daughter of your parent's sister or brother.
Your mother's niece is your first cousin. Your first cousin's daughter is your first cousin, once removed.
She would be your son's second cousin.
Since your niece is your daughter's first cousin, your grand niece (the daughter of your niece) is your daughter's first cousin, once removed.
Your grand-niece.
A niece. How could you not know that
cousin daughter
Your father's niece is your first cousin, and her daughter is your first cousin, once removed.
Your first cousin's niece is the daughter of her brother or sister. Your first cousin's brothers and sisters are also your first cousins. So your first cousin's niece is the daughter of your first cousin and is therefore your first cousin, once removed.
Either they are second cousins or technically not related at all. Your cousin's niece would be one of her siblings' children, in which case it would one of your cousin's daughters, or her spouses' niece, in which case there is no blood relationship, but most families would refer to them as a cousin.
Your niece is the first cousin of your children and the first cousin, once removed, of your grandchildren.
Your father's niece is your first cousin, and her daughter is your first cousin, once removed.