조카 (choka) means “niece or nephew”

More examples

주말에 조카를 만나러 갔어요.chu ma re cho ka reul man na reo ka sseo yo

I went to meet a niece or nephew over the weekend.

One family word for the child of either a brother or a sister.

사촌sachon: A child of an aunt or uncle in the same generation.
친척chincheok: A person connected to the family by birth or marriage.

형제나 자매에게서 태어난 아이를 가리키는 말hyeong je na cha mae e ge seo tae eo nan a i reul ka ri ki neun mal

This is a long-standing Korean family word with no clear modern parts, so remembering it as one word for a sibling's child is most useful.

This word prevents a common mix-up because it does not require choosing between the English words niece and nephew.