squill vs sea onion

squill and sea onion both are nouns.

Word	Noun	Adjective	Verb	Adverb
squill	Yes	No	No	No
sea onion	Yes	No	No	No

As nouns, sea onion is a hyponym of squill; that is, sea onion is a word with a more specific, narrower meaning than squill:

squill: an Old World plant of the genus Scilla having narrow basal leaves and pink or blue or white racemose flowers
sea onion: European scilla with small blue or purple flowers

Other hyponyms of squill include Scilla verna, spring squill.

As nouns, squill and sea onion are synonyms defined as:

squill and sea onion: having dense spikes of small white flowers and yielding a bulb with medicinal properties

Other synonyms of squill include sea squill, Urginea maritima.

squill (noun)	sea onion (noun)
an Old World plant of the genus Scilla having narrow basal leaves and pink or blue or white racemose flowers	European scilla with small blue or purple flowers
having dense spikes of small white flowers and yielding a bulb with medicinal properties	having dense spikes of small white flowers and yielding a bulb with medicinal properties
bulb of the sea squill, which is sliced, dried, and used as an expectorant