outside has 11 distinct synonyms spread across 8 of its 14 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 inside.
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.
relating to or being on or near the outer side or limit
“an outside margin”
coming from the outside
“extraneous light in the camera spoiled the photograph”
(of a baseball pitch) on the far side of home plate from the batter
“the pitch was away (or wide)”
4 of the 14 recorded senses carry no synonym, antonym or related term, so they are not shown above. Every sense, with its definition and examples, is on the full entry for outside.
Synonyms, antonyms and the broader and narrower relations for outside come from Princeton University’s WordNet. Full sources and licences.