孙子(sūn zi) - grandson (paternal family) in Mandarin

孙子 (sūn zi) is "grandson" specifically on the father's side - a son's son. Mandarin kinship terms split by which parent's side of the family the relative belongs to, so 孙子 (sūn zi) is distinct from 外孙 (wàisūn), the grandson through a daughter. English collapses both into one word, "grandson"; Mandarin keeps the family line visible in the vocabulary itself.

The second syllable 子 (zi) here is a neutral-tone suffix, not the full-toned noun 子 (zǐ, "child, son") it descends from - say 孙子 (sūn zi) with a light, toneless second syllable, the same pattern as 儿子 (érzi, "son") and 桌子 (zhuōzi, "table").

孙子 (sūn zi) pairs naturally with 孙女 (sūn nǚ, "granddaughter", paternal side) and with the grandparents who use the word: 爷爷 (yéye, "paternal grandfather") and 奶奶 (nǎinai, "paternal grandmother"), since a person's own 孙子 (sūn zi) is specifically their son's son.