子弹 (zǐ dàn) literally means 'child bullet' — but this is a historical compound where 子 functions as a diminutive or generic noun suffix (like in 花子 'blossom', 瓶子 'bottle'), and 弹 means 'projectile' or 'shell'. Together, they form the standard modern term for a small-caliber projectile fired from firearms such as pistols, rifles, or machine guns. It is a countable noun and commonly appears with measure words like 颗 (kē) or 发 (fā), e.g., 一颗子弹 ('one bullet') or 一发子弹 ('one round').

The word is used across military, law enforcement, medical, and news contexts — especially when discussing injuries, weapons, safety, or forensic reports. While it can appear in historical or literary texts, its primary usage today is technical and factual. Unlike more colloquial or slang terms (e.g., 子儿 for 'coin'), 子弹 carries no informal or humorous connotation; it is neutral and precise.