cost accounting vs accountancy

cost accounting and accountancy both are nouns.

Word	Noun	Adjective	Verb	Adverb
cost accounting	Yes	No	No	No
accountancy	Yes	No	No	No

As nouns, accountancy is a hypernym of cost accounting; that is, accountancy is a word with a broader meaning than cost accounting:

cost accounting: keeping account of the costs of items in production
accountancy: the occupation of maintaining and auditing records and preparing financial reports for a business

Other hypernyms of cost accounting include accounting.

cost accounting (noun)	accountancy (noun)
keeping account of the costs of items in production	the occupation of maintaining and auditing records and preparing financial reports for a business