The Collaborative International Dictionary of English v.0.48:

Copula \Cop"u*la\, n. [L., bond, band. See Couple.]
   1. (Logic & Gram.) The word which unites the subject and
      predicate.
      [1913 Webster]

   2. (Mus.) The stop which connects the manuals, or the manuals
      with the pedals; -- called also coupler.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Coupler \Coup"ler\ (k?p"l?r), n.
   One who couples; that which couples, as a link, ring, or
   shackle, to connect cars.
   [1913 Webster]

   Coupler of an organ, a contrivance by which any two or more
      of the ranks of keys, or keys and pedals, are connected so
      as to act together when the organ is played.
      [1913 Webster]

WordNet (r) 3.0 (2006):

coupler
    n 1: a mechanical device that serves to connect the ends of
         adjacent objects [syn: coupling, coupler]