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