thin out vs lessen

thin out and lessen both are verbs.

Word	Noun	Adjective	Verb	Adverb
thin out	No	No	Yes	No
lessen	No	No	Yes	No

As verbs, lessen is a hypernym of thin out; that is, lessen is a word with a broader meaning than thin out:

Other hypernyms of thin out include decrease, diminish, fall.

thin out (verb)	lessen (verb)
make sparse	decrease in size, extent, or range
become sparser	wear off or die down
lessen the strength or flavor of a solution or mixture	make become smaller