coin bank vs piggy bank

coin bank and piggy bank both are nouns.

Word	Noun	Adjective	Verb	Adverb
coin bank	Yes	No	No	No
piggy bank	Yes	No	No	No

As nouns, piggy bank is a hyponym of coin bank; that is, piggy bank is a word with a more specific, narrower meaning than coin bank:

coin bank: a container (usually with a slot in the top) for keeping money at home
piggy bank: a child's coin bank (often shaped like a pig)

Other hyponyms of coin bank include penny bank.

coin bank (noun)	piggy bank (noun)
a container (usually with a slot in the top) for keeping money at home	a child's coin bank (often shaped like a pig)