WordNet records no synonym and no antonym for backgrounder. It is the only member of its synset, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
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 press conference or interview in which a government official explains to reporters the background of an action or policy
“the secretary gave us a backgrounder on public health issues”
Synonyms, antonyms and the broader and narrower relations for backgrounder come from Princeton University’s WordNet. Full sources and licences.