plus has 4 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. Its one recorded opposite is minus.
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.
the arithmetic operation of summing; calculating the sum of two or more numbers
“the summation of four and three gives seven”
Synonyms, antonyms and the broader and narrower relations for plus come from Princeton University’s WordNet. Full sources and licences.