usurp vs hijack

usurp is not a noun while hijack is a noun.

Word	Noun	Adjective	Verb	Adverb
usurp	No	No	Yes	No
hijack	Yes	No	Yes	No

As verbs, hijack is a hyponym of usurp; that is, hijack is a word with a more specific, narrower meaning than usurp:

usurp: seize and take control without authority and possibly with force; take as one's right or possession
hijack: seize control of

Other hyponyms of usurp include annex, appropriate, capture, conquer, seize, preoccupy, raid.

usurp (noun)	hijack (noun)
	seizure of a vehicle in transit either to rob it or divert it to an alternate destination

usurp (verb)	hijack (verb)
seize and take control without authority and possibly with force; take as one's right or possession	take arbitrarily or by force
take the place of	seize control of