The Collaborative International Dictionary of English v.0.48:

Invocation \In`vo*ca"tion\, n. [F. invocation, L. invocatio.]
   [1913 Webster]
   1. The act or form of calling for the assistance or presence
      of some superior being; earnest and solemn entreaty; esp.,
      prayer offered to a divine being.
      [1913 Webster]

            Sweet invocation of a child; most pretty and
            pathetical!                           --Shak.
      [1913 Webster]

            The whole poem is a prayer to Fortune, and the
            invocation is divided between the two deities.
                                                  --Addison.
      [1913 Webster]

   2. (Law) A call or summons; especially, a judicial call,
      demand, or order; as, the invocation of papers or evidence
      into court.
      [1913 Webster]

WordNet (r) 3.0 (2006):

invocation
    n 1: a prayer asking God's help as part of a religious service
         [syn: invocation, supplication]
    2: an incantation used in conjuring or summoning a devil
    3: calling up a spirit or devil [syn: conjuring,
       conjuration, conjury, invocation]
    4: the act of appealing for help

Moby Thesaurus II by Grady Ward, 1.0:

92 Moby Thesaurus words for "invocation":
   Angelus, Ave, Ave Maria, Hail Mary, Kyrie Eleison, Paternoster,
   adjuration, agape, aid prayer, appeal, asperges, aspersion,
   auricular confession, bar mitzvah, bas mitzvah, beadroll, beads,
   beck, beck and call, benedicite, benediction, benison, beseechment,
   bid, bidding, bidding prayer, blessing, breviary, call, calling,
   calling forth, celebration, chaplet, circumcision, clamor, collect,
   communion, confession, confirmation, conjuration, conjurement,
   contemplation, convocation, cry, devotions, entreaty, evocation,
   exorcisation, exorcism, exsufflation, grace, high celebration,
   impetration, imploration, imploring, imprecation, incantation,
   incense, indent, intercession, invocation of saints,
   invocatory plea, kiss of peace, lesser litany, litany, love feast,
   lustration, meditation, nod, obsecration, obtestation, orison, pax,
   petition, plea, prayer, prayer wheel, preconization, processional,
   reciting the rosary, requisition, rogation, rosary, silent prayer,
   suit, summons, supplication, telling of beads, thanks,
   thanksgiving, the confessional, the confessionary