addition vs accretion

addition and accretion both are nouns.

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

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

addition (noun)	accretion (noun)
the arithmetic operation of summing; calculating the sum of two or more numbers	(law) an increase in a beneficiary's share in an estate (as when a co-beneficiary dies or fails to meet some condition or rejects the inheritance)
the act of adding one thing to another	an increase by natural growth or addition
a component that is added to something to improve it	(geology) an increase in land resulting from alluvial deposits or waterborne sediment
a suburban area laid out in streets and lots for a future residential area	(biology) growth by addition as by the adhesion of parts or particles
something added to what you already have	(astronomy) the formation of a celestial object by the effect of gravity pulling together surrounding objects and gases
a quantity that is added	something contributing to growth or increase