buffer has 10 distinct synonyms spread across 7 of its 9 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.
(chemistry) an ionic compound that resists changes in its pH
a neutral zone between two rival powers that is created in order to diminish the danger of conflict
an inclined metal frame at the front of a locomotive to clear the track
(computer science) a part of RAM used for temporary storage of data that is waiting to be sent to a device; used to compensate for differences in the rate of flow of data between components of a computer system
add a buffer (a solution)
“buffered saline solution for the eyes”
Synonyms, antonyms and the broader and narrower relations for buffer come from Princeton University’s WordNet. Full sources and licences.