The Collaborative International Dictionary of English v.0.48:

Summation \Sum*ma"tion\, n. [Cf. F. sommation. See Sum, v. t.]
   The act of summing, or forming a sum, or total amount; also,
   an aggregate.
   [1913 Webster]

         Of this series no summation is possible to a finite
         intellect.                               --De Quincey.
   [1913 Webster]

WordNet (r) 3.0 (2006):

summation
    n 1: a concluding summary (as in presenting a case before a law
         court) [syn: summation, summing up, rundown]
    2: (physiology) the process whereby multiple stimuli can produce
       a response (in a muscle or nerve or other part) that one
       stimulus alone does not produce
    3: the final aggregate; "the sum of all our troubles did not
       equal the misery they suffered" [syn: sum, summation,
       sum total]
    4: the arithmetic operation of summing; calculating the sum of
       two or more numbers; "the summation of four and three gives
       seven"; "four plus three equals seven" [syn: summation,
       addition, plus]

Moby Thesaurus II by Grady Ward, 1.0:

77 Moby Thesaurus words for "summation":
   abbreviation, abridgment, abstract, account, accounts, addend,
   aggregate, amount, apocope, argument, body count, box score,
   capitulation, cast, census, compression, condensation, conspectus,
   core, count, curtailment, difference, elision, ellipsis, epitome,
   essence, evidence, foreshortening, gist, head count, inventory,
   main point, meat, nose count, number, pith, plus, plus sign,
   precis, product, quantity, recap, recapitulation, reckoning,
   recount, recounting, reduction, rehearsal, repertory, resume,
   retrenchment, run-through, rundown, score, shortening, statement,
   substance, subtotal, sum, sum and substance, summary, summing,
   summing up, summing-up, syncope, synopsis, tale, tally,
   telescoping, testimony, the bottom line, the story,
   the whole story, total, truncation, whole, x number