pseudoscorpion vs arachnid

pseudoscorpion and arachnid both are nouns.

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

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

pseudoscorpion: small nonvenomous arachnid resembling a tailless scorpion
arachnid: air-breathing arthropods characterized by simple eyes and four pairs of legs

Other hypernyms of pseudoscorpion include arachnoid.

pseudoscorpion (noun)	arachnid (noun)
small nonvenomous arachnid resembling a tailless scorpion	air-breathing arthropods characterized by simple eyes and four pairs of legs