Two lines of poetry that sit together as a pair — usually the same length, and usually rhyming. Poets use a couplet to finish something off: a Shakespeare sonnet runs for twelve lines and then snaps shut with a couplet, and that little pair of rhymes is what makes the ending feel like an ending.

Strictly speaking, rhyme isn't required — two paired lines are a couplet whether they rhyme or not — but in ordinary use, if someone says couplet, expect rhyme. It's mostly a classroom and literary word; you're unlikely to hear it at dinner.

One thing that helps Taiwanese students: this is very close to 對聯, the pair of red banners at a doorway at New Year. Same idea of two matched lines that answer each other. But don't treat them as the same thing — 對聯 must match in tone and in word class position by position, which English couplets never do, and English couplets rhyme, which 對聯 don't. Similar spirit, different rules. Translators of Chinese poetry often use couplet for 對句, so you'll see the word used that way too.