fiddlehead has 8 distinct synonyms spread across 2 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.
tall fern of northern temperate regions having graceful arched fronds and sporophylls resembling ostrich plumes
New World fern having woolly cinnamon-colored spore-bearing fronds in early spring later surrounded by green fronds; the early uncurling fronds are edible
Synonyms, antonyms and the broader and narrower relations for fiddlehead come from Princeton University’s WordNet. Full sources and licences.