a son of your brother or sister
"My nephew is three years younger than me and just learned how to ride a bike without training wheels."
A son of one's sibling, brother-in-law, or sister-in-law; either a son of one's brother (fraternal nephew) or a son of one's sister (sororal nephew).
"When my aunt gave me the old family heirloom, I realized it was being passed down to her only surviving nephew."
A surname.
"After years of searching, the historian finally located a letter signed by Nephew during his time in colonial Virginia."
The word "nephew" comes from the Latin nepos, meaning "nephew or grandson," which entered English through Old French. It replaced an older Germanic term of similar meaning that had been used in English since before the Norman Conquest.