mission has 10 distinct synonyms spread across 5 of its 5 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.
an organization of missionaries in a foreign land sent to carry on religious work
an operation that is assigned by a higher headquarters
“the planes were on a bombing mission”
a special assignment that is given to a person or group
“a confidential mission to London”
a group of representatives or delegates
Synonyms, antonyms and the broader and narrower relations for mission come from Princeton University’s WordNet. Full sources and licences.