adjustment vs improvement

adjustment and improvement both are nouns.

Word	Noun	Adjective	Verb	Adverb
adjustment	Yes	No	No	No
improvement	Yes	No	No	No

As nouns, improvement is a hypernym of adjustment; that is, improvement is a word with a broader meaning than adjustment:

Other hypernyms of adjustment include advance, betterment.

adjustment (noun)	improvement (noun)
the act of making something different (as e.g. the size of a garment)	the act of improving something
the act of adjusting something to match a standard	a change for the better; progress in development
making or becoming suitable; adjusting to circumstances	a condition superior to an earlier condition
an amount added or deducted on the basis of qualifying circumstances
the process of adapting to something (such as environmental conditions)