The Collaborative International Dictionary of English v.0.48:

Summon \Sum"mon\, v. t. [imp. & p. p. Summoned; p. pr. & vb.
   n. Summoning.] [OE. somonen, OF. sumundre, semondre, F.
   semondre, from (assumed) LL. summon[e^]re, for L.
   summon[=e]re to give a hint; sub under + monere to admonish,
   to warn. See Monition, and cf. Submonish.]
   1. To call, bid, or cite; to notify to come to appear; --
      often with up.
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            Stiffen the sinews, summon up the blood. --Shak.
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            Trumpets summon him to war.           --Dryden.
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   2. To give notice to, or command to appear, as in court; to
      cite by authority; as, to summon witnesses.
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   3. (Mil.) To call upon to surrender, as a fort.
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   Syn: To call; cite; notify; convene; convoke; excite; invite;
        bid. See Call.
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WordNet (r) 3.0 (2006):

summon
    v 1: call in an official matter, such as to attend court [syn:
         summon, summons, cite]
    2: ask to come; "summon a lawyer"
    3: gather or bring together; "muster the courage to do
       something"; "she rallied her intellect"; "Summon all your
       courage" [syn: muster, rally, summon, come up,
       muster up]
    4: make ready for action or use; "marshal resources" [syn:
       mobilize, mobilise, marshal, summon]