summation has 6 distinct synonyms spread across 3 of its 4 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a concluding summary (as in presenting a case before a law court)
(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
Synonyms, antonyms and the broader and narrower relations for summation come from Princeton University’s WordNet. Full sources and licences.
the final aggregate
“the sum of all our troubles did not equal the misery they suffered”
the arithmetic operation of summing; calculating the sum of two or more numbers
“the summation of four and three gives seven”