mend vs improvement

mend and improvement both are nouns.

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

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

mend (noun)	improvement (noun)
the act of putting something in working order again	the act of improving something
sewing that repairs a worn or torn hole (especially in a garment)	a change for the better; progress in development
	a condition superior to an earlier condition

mend (verb)	improvement (verb)
restore by replacing a part or putting together what is torn or broken
heal or recover