From BiblePortal Wikipedia

Webster's Dictionary [1]

(n.) A nuptial song, or poem in honor of the bride and bridegroom.

The Nuttall Encyclopedia [2]

A nuptial song, sung before the bridal chamber in honour of the newly-wedded couple, particularly among the Greeks and Romans, of whom Theocritus and Catullus have left notable examples; but the epithalamium of Edmund Spenser is probably the finest specimen extant of this poetic form.