(someone else's) niece (honorific)
Honorific term used to refer to another person's niece, showing respect toward that person or their family. Not used for one's own niece.
I hear the company president's niece is going to study abroad next year.
How old is your niece?
姪御 is a respectful term used to refer to someone else's niece, while 姪 is the plain, neutral word usable for anyone's niece.
Compound of 姪 (mei, 'niece') and the honorific suffix 御 (go).