law vs tax law

law and tax law both are nouns.

Word	Noun	Adjective	Verb	Adverb
law	Yes	No	No	No
tax law	Yes	No	No	No

As nouns, tax law is a hyponym of law; that is, tax law is a word with a more specific, narrower meaning than law:

law (noun)	tax law (noun)
the learned profession that is mastered by graduate study in a law school and that is responsible for the judicial system	the body of laws governing taxation
a rule or body of rules of conduct inherent in human nature and essential to or binding upon human society
a generalization that describes recurring facts or events in nature
the branch of philosophy concerned with the law and the principles that lead courts to make the decisions they do
legal document setting forth rules governing a particular kind of activity
the collection of rules imposed by authority
the force of policemen and officers