let up vs diminish

let up and diminish both are verbs.

Word	Noun	Adjective	Verb	Adverb
let up	No	No	Yes	No
diminish	No	No	Yes	No

As verbs, diminish is a hypernym of let up; that is, diminish is a word with a broader meaning than let up:

Other hypernyms of let up include decrease, fall, lessen.

let up (verb)	diminish (verb)
become less in amount or intensity	decrease in size, extent, or range
reduce pressure or intensity	lessen the authority, dignity, or reputation of