lengthening vs continuation

lengthening and continuation both are nouns.

Word	Noun	Adjective	Verb	Adverb
lengthening	Yes	No	No	No
continuation	Yes	No	No	No

As nouns, continuation is a hypernym of lengthening; that is, continuation is a word with a broader meaning than lengthening:

Other hypernyms of lengthening include continuance.

lengthening (noun)	continuation (noun)
the act of prolonging something	the act of continuing an activity without interruption
	the consequence of being lengthened in duration
	a Gestalt principle of organization holding that there is an innate tendency to perceive a line as continuing its established direction
	a part added to a book or play that continues and extends it