Why do multiple countries share "Dollar" as the same currency name?

There are about 30 countries that share the same currency name "Dollar" (USA, Canada, Australia, New Zealand, Brunei, Singapore, Barbados, Fiji, Cayman Islands & etc.). but they are not related to each other besides their name, since they have different values from one another, even though they share the same currency symbol: only thing that distinguishes them apart is the country prefix tied to $ (like US$, AU$, NZ$, C$, CI$, FJ$, SG$, Bds$, JA$, B$, etc.).