独生子(dú shēng zǐ) - only child, only son in Mandarin

独生子 (dúshēngzǐ) is an only son, though in loose everyday use people often stretch it to mean "only child" in general; the strictly gender-neutral terms are 独生女 (dúshēngnǚ, only daughter) and 独生子女 (dúshēngzǐnǚ, only child of either sex). Break it down and it reads 独 (dú, alone, only) + 生 (shēng, born) + 子 (zǐ, son, child).

The word is inseparable from China's one-child policy, in force from 1979 to 2015, so most people born in that window describe themselves this way, and the generation has its own reputation in the culture for being both indulged and lonely.

Watch the tone on 子 (zǐ): here it keeps its full third tone because it means "child, son" as a real content word, unlike the grammatical suffix 子 (zi) that turns 桌 (zhuō, table) into 桌子 (zhuōzi) with a flat neutral tone.