addition vs step-up

addition and step-up both are nouns.

Word	Noun	Adjective	Verb	Adverb
addition	Yes	No	No	No
step-up	Yes	No	No	No

As nouns, step-up is a hypernym of addition; that is, step-up is a word with a broader meaning than addition:

Other hypernyms of addition include increase.

addition (noun)	step-up (noun)
the arithmetic operation of summing; calculating the sum of two or more numbers	the act of increasing something
the act of adding one thing to another
a component that is added to something to improve it
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