flapping vs undulation

flapping and undulation both are nouns.

Word	Noun	Adjective	Verb	Adverb
flapping	Yes	No	No	No
undulation	Yes	No	No	No

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

Other hypernyms of flapping include wave.

flapping (noun)	undulation (noun)
the motion made by flapping up and down	(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