A couplet is two successive lines of poetry, often rhymed.
From A Poet’s Glossary
The following definition of the term couplet is reprinted from A Poet's Glossary by Edward Hirsch.
The couplet, two successive lines of poetry, usually rhymed (aa), has been an elemental stanzaic unit—a couple, a pairing—as long as there has been written rhyming poetry in English. It can stand as an epigrammatic poem on its own, a weapon for aphoristic wit, as in Pope’s “Epigram Engraved on the Collar of a Dog which I gave to his Royal Highness” (1734):
I am his Highness’ Dog at Kew;
Pray tell me Sir, whose Dog are you?
The couplet also serves as an organizing pattern in long poems (Shakespeare’s “Venus and Adonis,” 1592–1593; Marlowe’s “Hero and Leander,” 1593) or part of a larger stanzaic unit. It stands as the pithy conclusion to the ottava rima stanza (abababcc), the rhyme royal stanza (ababbcc), and the Shakespearean sonnet (ababcdcdefefgg).
The rhyming iambic pentameter or five-stress couplet—later known as the heroic couplet—was introduced into English by Chaucer in “The Prologue to the Legend of Good Women” (1386), in imitation of French meter, and employed for most of The Canterbury Tales (ca. 1387–1400). It has sometimes been nicknamed riding rhyme, probably because the pilgrims reeled them off while they were riding to Canterbury. It was taken up and used with great flexibility by the Tudor and Jacobean poets and dramatists. Nicholas Grimald’s pioneering experiments with the heroic couplet should be better known (Tottel’s Miscellany, 1557). Christopher Marlowe employed the heroic couplet for his daring translation of Ovid’s Amores (16 B.C.E.), which he called Ovid’s Elegies (1594–1595). The mighty two-liner was also used by William Shakespeare, George Chapman, and John Donne, and then stamped as a neoclassical form by John Dryden, Alexander Pope, and Samuel Johnson, who wrote:
Let Observation with extensive View
Survey Mankind, from China to Peru . . .
This closed form of the couplet is well suited to express aphoristic wit.
The octosyllabic or four-stress couplet, probably based on a common Latin meter, became a staple of English medieval verse (such as The Lay of Havelok the Dane, ca. 1280–1290), then was virtually reinvented by Samuel Butler in his mock-heroic satire Hudibras (1663–1680), whose couplets became known as Hudibrastics, and raised to a higher power by Milton (“L’Allegro” and “II Penseroso,” both 1645), Marvell (“To His Coy Mistress,” ca. 1650s), and Coleridge (“Christabel,” 1797–1800).
We call a couplet closed when the sense and syntax come to a conclusion or strong pause at the end of the second line, thus giving a feeling of self-containment and enclosure, as in the first lines of “To His Coy Mistress”:
Had we but world enough, and time,
This coyness, Lady, were no crime.
We call a couplet open when the sense carries forward past the second line into the next line or lines, as in the beginning of Keats’s Endymion (1818):
A thing of beauty is a joy for ever:
Its loveliness increases; it will never
Pass into nothingness, but still will keep
A bower quiet for us, and a sleep
Full of sweet dreams . . .
Ben Jonson told William Drummond that he deemed couplets “the bravest Sort of Verses, especially when they are broken.” All two-line stanzas in English carry the vestigial memory of closed or open couplets.