system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How big does a landmass have to be for it to no longer be considered an island?
gpt
It has to be "sub-continental", basically, not a continent itself.

They generally define it based on what chunk of earths crust it sits on, called a "tectonic plate".  Ocean plates look different then continental plates.  If it's surrounded by water and isn't a continental plate, then it's an island, or if it's on a continental plate but separate from the main portion by water, it's an island.