成人(chéng rén) - adult, grown-up in Mandarin

成人 (chéngrén) means "adult" or "grown-up", and it can also work as a verb, "to become an adult", in the set phrase 长大成人 (zhǎngdà chéngrén), "to grow up into an adult". 成 (chéng) means "to become" and 人 (rén) means "person", so the compound reads plainly as "become a person".

In everyday speech 大人 (dàrén) is the word you reach for when talking to or about a specific grown-up, often from a child's point of view, while 成人 (chéngrén) is the more formal, categorical term you see printed on signs: 成人票 (chéngrén piào, adult ticket), 成人教育 (chéngrén jiàoyù, adult education).

An English speaker's trap is letting the tone slip on the second syllable out of habit from other compounds; both syllables here carry a level, rising second tone, 成 (chéng) and 人 (rén), with no dip or fall anywhere in the word.