addition vs elongation

addition and elongation both are nouns.

Word	Noun	Adjective	Verb	Adverb
addition	Yes	No	No	No
elongation	Yes	No	No	No

As nouns, elongation is a hyponym of addition; that is, elongation is a word with a more specific, narrower meaning than addition:

Other hyponyms of addition include additive, afterthought, annex, annexe, extension, wing, attachment, extension.

addition (noun)	elongation (noun)
the arithmetic operation of summing; calculating the sum of two or more numbers	the act of lengthening something
the act of adding one thing to another	an addition to the length of something
a component that is added to something to improve it	the quality of being elongated
a suburban area laid out in streets and lots for a future residential area
something added to what you already have
a quantity that is added