further has 6 distinct synonyms spread across 5 of its 6 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.
to or at a greater extent or degree or a more advanced stage (`further' is used more often than `farther' in this abstract sense)
“further complicated by uncertainty about the future”
to or at a greater distance in time or space (`farther' is used more frequently than `further' in this physical sense)
“farther north”
1 of the 6 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 further.
Synonyms, antonyms and the broader and narrower relations for further come from Princeton University’s WordNet. Full sources and licences.