gravity wave vs undulation

gravity wave and undulation both are nouns.

Word	Noun	Adjective	Verb	Adverb
gravity wave	Yes	No	No	No
undulation	Yes	No	No	No

As nouns, undulation is a hypernym of gravity wave; that is, undulation is a word with a broader meaning than gravity wave:

gravity wave: (physics) a wave that is hypothesized to propagate gravity and to travel at the speed of light
undulation: (physics) a movement up and down or back and forth

Other hypernyms of gravity wave include wave.

gravity wave (noun)	undulation (noun)
(physics) a wave that is hypothesized to propagate gravity and to travel at the speed of light	(physics) a movement up and down or back and forth
	wavelike motion; a gentle rising and falling in the manner of waves
	an undulating curve