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]

WordNet (r) 3.0 (2006):

copula
    n 1: an equating verb (such as `be' or `become') that links the
         subject with the complement of a sentence [syn: copula,
         copulative, linking verb]