The Collaborative International Dictionary of English v.0.48:

Couplet \Coup"let\ (-l?t), n. [F. couplet, dim. of couple. See
   Couple, n. ]
   Two taken together; a pair or couple; especially two lines of
   verse that rhyme with each other.
   [1913 Webster]

         A sudden couplet rushes on your mind.    --Crabbe.
   [1913 Webster]

WordNet (r) 3.0 (2006):

couplet
    n 1: two items of the same kind [syn: couple, pair,
         twosome, twain, brace, span, yoke, couplet,
         distich, duo, duet, dyad, duad]
    2: a stanza consisting of two successive lines of verse; usually
       rhymed

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "couplet":
   Spenserian stanza, antistrophe, book, both, brace, burden, canto,
   chorus, couple, distich, double harness, doublet, duad, duet, duo,
   dyad, envoi, epode, heptastich, hexastich, line, match, mates,
   measure, monostich, octastich, octave, octet, ottava rima, pair,
   pentastich, quatrain, refrain, rhyme royal, septet, sestet,
   set of two, sextet, span, stanza, stave, strain, strophe, syllable,
   team, tercet, terza rima, tetrastich, the two, triplet, tristich,
   twain, two, twosome, verse, yoke