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]