The Collaborative International Dictionary of English v.0.48:

Mission \Mis"sion\, n. [L. missio, fr. mittere, missum, to send:
   cf. F. mission. See Missile.]
   1. The act of sending, or the state of being sent; a being
      sent or delegated by authority, with certain powers for
      transacting business; comission.
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            Whose glorious deeds, but in these fields of late,
            Made emulous missions 'mongst the gods themselves.
                                                  --Shak.
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   2. That with which a messenger or agent is charged; an
      errand; business or duty on which one is sent; a
      commission.
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            How to begin, how to accomplish best
            His end of being on earth, and mission high.
                                                  --Milton.
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   3. Persons sent; any number of persons appointed to perform
      any service; a delegation; an embassy; as, the Russian
      mission to the United Nations.
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            In these ships there should be a mission of three of
            the fellows or brethren of Solomon's house. --Bacon.
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   4. An assotiation or organization of missionaries; a station
      or residence of missionaries.
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   5. An organization for worship and work, dependent on one or
      more churches.
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   6. A course of extraordinary sermons and services at a
      particular place and time for the special purpose of
      quickening the faith and zeal participants, and of
      converting unbelievers. --Addis & Arnold.
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   7. Dismission; discharge from service. [Obs.]
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   Mission school.
      (a) A school connected with a mission and conducted by
          missionaries.
      (b) A school for the religious instruction of children not
          having regular church privileges.
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   Syn: Message; errand; commission; deputation.
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The Collaborative International Dictionary of English v.0.48:

Mission \Mis"sion\, v. t.
   To send on a mission. [Mostly used in the form of the past
   participle.] --Keats.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mission
    n 1: an organization of missionaries in a foreign land sent to
         carry on religious work [syn: mission, missionary post,
         missionary station, foreign mission]
    2: an operation that is assigned by a higher headquarters; "the
       planes were on a bombing mission" [syn: mission, military
       mission]
    3: a special assignment that is given to a person or group; "a
       confidential mission to London"; "his charge was deliver a
       message" [syn: mission, charge, commission]
    4: the organized work of a religious missionary [syn: mission,
       missionary work]
    5: a group of representatives or delegates [syn: deputation,
       commission, delegation, delegacy, mission]