triumphal arch vs memorial

triumphal arch and memorial both are nouns.

Word	Noun	Adjective	Verb	Adverb
triumphal arch	Yes	No	No	No
memorial	Yes	No	No	No

As nouns, memorial is a hypernym of triumphal arch; that is, memorial is a word with a broader meaning than triumphal arch:

triumphal arch: a monumental archway; usually they are built to commemorate some notable victory
memorial: a structure erected to commemorate persons or events

Other hypernyms of triumphal arch include arch, monument.

triumphal arch (noun)	memorial (noun)
a monumental archway; usually they are built to commemorate some notable victory	a structure erected to commemorate persons or events
	a recognition of meritorious service
	a written statement of facts submitted in conjunction with a petition to an authority