down has 38 distinct synonyms spread across 13 of its 26 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 up.
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.
being or moving lower in position or less in some value
“lay face down”
filled with melancholy and despondency
“gloomy at the thought of what he had to face”
eat up completely, as with great appetite
“Some people can down a pound of meat in the course of one meal”
bring down or defeat (an opponent)
spatially or metaphorically from a higher to a lower level or position
“don't fall down”
away from a more central or a more northerly place
“was sent down to work at the regional office”
to a lower intensity
“he slowly phased down the light until the stage was completely black”
(American football) a complete play to advance the football
“you have four downs to gain ten yards”
English physician who first described Down's syndrome (1828-1896)
(usually plural) a rolling treeless highland with little soil
7 of the 26 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 down.
Synonyms, antonyms and the broader and narrower relations for down come from Princeton University’s WordNet. Full sources and licences.