The Collaborative International Dictionary of English v.0.48:

Epithalamium \Ep`i*tha*la"mi*um\, n.; pl. Epithalamiums, L.
   Epithalamia. [L., fr. Gr. ?, orig. an adj., nuptial; 'epi`
   upon, at + ? bride chamber.]
   A nuptial song, or poem in honor of the bride and bridegroom.
   [1913 Webster]

         The kind of poem which was called epithalamium . . .
         sung when the bride was led into her chamber. --B.
                                                  Jonson.
   [1913 Webster]

WordNet (r) 3.0 (2006):

epithalamium
    n 1: an ode honoring a bride and bridegroom