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.
      [1913 Webster]

            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.
        [1913 Webster]

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]

Moby Thesaurus II by Grady Ward, 1.0:

109 Moby Thesaurus words for "summon":
   appeal, arouse, ask, assemble, attract, be attractive, beckon, bid,
   bid come, bring forth, bring out, bring to light, call, call away,
   call back, call for, call forth, call in, call out, call together,
   call up, call up spirits, cite, command, commandeer, conjure,
   conjure up, conjure up spirits, conscript, convene, convoke,
   deduce, demand, derive, detach, detach for service, draft,
   drag out, draw forth, draw out, draw up, educe, elicit, engage,
   enjoin, enlist, enroll, evoke, fetch, gather, get from, get out of,
   get together, hold a meeting, hold a session, impress, indent,
   induce, induct, interest, invite, invoke, issue an invitation,
   join, levy, list, meet, mobilize, muster, muster in, muster up,
   obtain, order, order up, page, preconize, press, procure, raise,
   raise ghosts, recall, recruit, requisition, rouse, secure,
   send after, send for, serve, sign on, sign up, sit, stimulate,
   subpoena, summon forth, summon spirits, summon up, summons,
   tantalize, tease, tempt, tickle, titillate, wangle, wangle out of,
   whet the appetite, winkle out, work up, worm out, worm out of