One's brother's or sister's son; nephew (colloquial).
うちの甥っ子は小学生で、サッカーが大好きだ。
My nephew is in elementary school and loves soccer.
お正月に甥っ子にお年玉をあげた。
I gave my nephew New Year's money at the New Year.
甥っ子が遊びに来たので、一緒に公園へ行った。
My nephew came to visit, so we went to the park together.