The Collaborative International Dictionary of English v.0.48:

Polyhymnia \Pol`y*hym"ni*a\, n. [L., from Gr. ?; poly`s many + ?
   hymn.] (Anc. Myth.)
   The Muse of lyric poetry.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Polyhymnia
    n 1: (Greek mythology) the Muse of singing and mime and sacred
         dance

Moby Thesaurus II by Grady Ward, 1.0:

42 Moby Thesaurus words for "Polyhymnia":
   Apollo, Apollo Musagetes, Bragi, Calliope, Castilian Spring, Clio,
   Erato, Euterpe, Helicon, Hippocrene, Melpomene, Muse, Orpheus,
   Parnassus, Pierian Spring, Pierides, Polymnia, Terpsichore, Thalia,
   afflatus, artistic imagination, conception, creative imagination,
   creative power, creative thought, esemplastic imagination,
   esemplastic power, fire of genius, genius, inspiration, muse,
   mythicization, mythification, mythopoeia, poesy, poetic genius,
   poetic imagination, sacred Nine, shaping imagination, the Muses,
   the Nine, tuneful Nine