bridgehead has 1 distinct synonym spread across 1 of its 2 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.
an area in hostile territory that has been captured and is held awaiting further troops and supplies
“an attempt to secure a bridgehead behind enemy lines”
a defensive post at the end of a bridge nearest to the enemy
Synonyms, antonyms and the broader and narrower relations for bridgehead come from Princeton University’s WordNet. Full sources and licences.