甥

In everyday speech 甥 is bound - you almost never see it solo. The common compound is 外甥 (sister's son), with the female counterpart 外甥女 (sister's daughter). The 'wài' (outside) marks them as the children of a married-out daughter - the patriarchal kinship system treats them as belonging to another family. A brother's son, by contrast, is 侄子 - INSIDE the lineage.

Classical kinship pairs the term with 舅 (maternal uncle): 舅甥 is the canonical uncle-nephew pair, while 叔侄 covers the paternal uncle and his 侄子. Knowing which uncle gets which nephew word is a small but real test of fluency in family talk.