asdic has 2 distinct synonyms spread across 1 of its 1 sense. 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 measuring instrument that sends out an acoustic pulse in water and measures distances in terms of the time for the echo of the pulse to return
“sonar is an acronym for sound navigation ranging”
Synonyms, antonyms and the broader and narrower relations for asdic come from Princeton University’s WordNet. Full sources and licences.