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