The Collaborative International Dictionary of English v.0.48:

Hymnody \Hym"no*dy\, n. [Gr. ?; ? a hymn + ? a song, a singing.]
   Hymns, considered collectively; hymnology.
   [1913 Webster]

WordNet (r) 3.0 (2006):

hymnody
    n 1: the act of singing psalms or hymns [syn: psalmody,
         hymnody]