tax deduction vs write-off

tax deduction and write-off both are nouns.

Word	Noun	Adjective	Verb	Adverb
tax deduction	Yes	No	No	No
write-off	Yes	No	No	No

As nouns, write-off is a hypernym of tax deduction; that is, write-off is a word with a broader meaning than tax deduction:

tax deduction: a reduction in the gross amount on which a tax is calculated; reduces taxes by the percentage fixed for the taxpayer's income bracket
write-off: (accounting) reduction in the book value of an asset

Other hypernyms of tax deduction include write-down.

tax deduction (noun)	write-off (noun)
a reduction in the gross amount on which a tax is calculated; reduces taxes by the percentage fixed for the taxpayer's income bracket	the act of cancelling from an account a bad debt or a worthless asset
	(accounting) reduction in the book value of an asset