This is one of the more asked around questions here.

I myself tackled it here, and here is another post by /u/itsalrightwithme, another good one by /u/Zhumanchu, also one by /u/drobb

In short, Amerigo Vespucci has claimed that South America is a new, previously unknown continent. Still, it remained possible that Caribbean islands and North America were in or near Asia. After Magellan's expedition, as well as with Portuguese presence in Asia, it became clear they are not close. Diego Ribeira's 1529 map is one of the first to definitely show them seperate continents divided by massive sea