scorpion vs arachnid

scorpion and arachnid both are nouns.

Word	Noun	Adjective	Verb	Adverb
scorpion	Yes	No	No	No
arachnid	Yes	No	No	No

As nouns, arachnid is a hypernym of scorpion; that is, arachnid is a word with a broader meaning than scorpion:

scorpion: arachnid of warm dry regions having a long segmented tail ending in a venomous stinger
arachnid: air-breathing arthropods characterized by simple eyes and four pairs of legs

Other hypernyms of scorpion include arachnoid.

scorpion (noun)	arachnid (noun)
arachnid of warm dry regions having a long segmented tail ending in a venomous stinger	air-breathing arthropods characterized by simple eyes and four pairs of legs