write down vs expense

write down is not a noun while expense is a noun.

Word	Noun	Adjective	Verb	Adverb
write down	No	No	Yes	No
expense	Yes	No	Yes	No

As verbs, write down and expense are synonyms defined as:

Other synonyms of write down include write off.

write down (noun)	expense (noun)
	a detriment or sacrifice
	amounts paid for goods and services that may be currently tax deductible (as opposed to capital expenditures)
	money spent to perform work and usually reimbursed by an employer

write down (verb)	expense (verb)
reduce the estimated value of something	reduce the estimated value of something
put down in writing; of texts, musical compositions, etc.