bottom has 24 distinct synonyms spread across 7 of its 12 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 2 recorded opposites are top, side.
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 lower side of anything
the lowest part of anything
“they started at the bottom of the hill”
the second half of an inning; while the home team is at bat
a depression forming the ground under a body of water
“he searched for treasure on the ocean bed”
1 of the 12 recorded senses carries no synonym, antonym or related term, so it is not shown above. Every sense, with its definition and examples, is on the full entry for bottom.
Synonyms, antonyms and the broader and narrower relations for bottom come from Princeton University’s WordNet. Full sources and licences.