medium has 7 distinct synonyms spread across 5 of its 13 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.
a means or instrumentality for storing or communicating information
the surrounding environment
“fish require an aqueous medium”
an intervening substance through which signals can travel as a means for communication
(bacteriology) a nutrient substance (solid or liquid) that is used to cultivate micro-organisms
(biology) a substance in which specimens are preserved or displayed
a state that is intermediate between extremes; a middle position
“a happy medium”
someone who serves as an intermediary between the living and the dead
“he consulted several mediums”
(usually plural) transmissions that are disseminated widely to the public
an occupation for which you are especially well suited
“in law he found his true metier”
1 of the 13 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 medium.
Synonyms, antonyms and the broader and narrower relations for medium come from Princeton University’s WordNet. Full sources and licences.