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

a nuptual song at marriages

a poem written to commemorate a wedding

A poem in honor of a bride and bridegroom.

Epithalamium (from Greek; epi- upon, and thalamium nuptial chamber, sometimes also spelled "epithalamion") specifically refers to a form of poem that is written for the bride. Or, specifically, written for the bride on the way to her marital chamber.