outboard has 2 distinct synonyms spread across 2 of its 3 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 inboard.
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.
internal-combustion engine that mounts at stern of small boat
Synonyms, antonyms and the broader and narrower relations for outboard come from Princeton University’s WordNet. Full sources and licences.
located away from the midline of a vessel or aircraft
“the outboard section of a wing”