WordNet records no synonym and no antonym for bay of biscay. 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.
an arm of the Atlantic Ocean in western Europe; bordered by the west coast of France and the north coast of Spain
Synonyms, antonyms and the broader and narrower relations for bay of biscay come from Princeton University’s WordNet. Full sources and licences.