Dealing with the constitutional, common-law, statutory, tax treaty, and regulatory rules that constitute the law applicable to taxation.

Tax law is an area of legal study dealing with the constitutional, common-law, statutory, tax treaty, and regulatory rules that constitute the law applicable to taxation. https://en.wikipedia.org/wiki/Tax_law

A governmental assessment upon property value, transactions, estates of the deceased, licenses granting a right and/or income, and duties on imports from foreign countries. It includes all contributions imposed by the government upon individuals for the service of the state.

Further reading: https://www.hg.org/tax.html