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”
on the positive side or higher end of a scale
“a plus value”
Synonyms, antonyms and the broader and narrower relations for plus come from Princeton University’s WordNet. Full sources and licences.