According to the website World Population Review, "an island country is a country that is made up of one or more islands or land that is surrounded completely by water."
There are approximately 670,000 islands in the world, but only 47 of them are technically countries. It takes next-level nation-knowledge to get them all, but let's see how you get on!
Oh, and sorry Aussies, your country is technically a continent, so that doesn't count as an island!