mediate has 6 distinct synonyms spread across 2 of its 4 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 immediate.
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.
act between parties with a view to reconciling differences
“He interceded in the family dispute”
occupy an intermediate or middle position or form a connecting link or stage between two others
“mediate between the old and the new”
Synonyms, antonyms and the broader and narrower relations for mediate come from Princeton University’s WordNet. Full sources and licences.
acting through or dependent on an intervening agency
“the disease spread by mediate as well as direct contact”
being neither at the beginning nor at the end in a series
“adolescence is an awkward in-between age”