The Collaborative International Dictionary of English v.0.48:

Turn-buckle \Turn"-buc`kle\ (t[^u]rn"b[u^]k`k'l), n. (Mech.)
   (a) A loop or sleeve with a screw thread at one end and a
       swivel at the other, -- used for tightening a rod, stay,
       etc.
   (b) A gravitating catch, as for fastening a shutter, the end
       of a chain, or a hasp.
       [1913 Webster]

WordNet (r) 3.0 (2006):

turnbuckle
    n 1: an oblong metal coupling with a swivel at one end and an
         internal thread at the other into which a threaded rod can
         be screwed in order to form a unit that can be adjusted for
         length or tension