How is this formally defined?
Well, it’s not formally defined, really.
A continent is a “large landmass” on Earth, but opinions on the specifics vary. In the United States, we’re taught that there are seven continents, but not everyone agrees. In different places and different time periods, the count has changed.
Many countries and many geologists currently combine Europe and Asia into “Eurasia.” In that line of thinking, they are separate continents only for arbitrary political and social reasons, not for any scientific reasons. Students in many countries are taught that there are only six continents.
North America and South America are sometimes combined as simply “America.” This was common in the earlier 20th century.
Europe, Asia, and Africa have sometimes been combined into “Afro-Eurasia”
“Oceania” has sometimes been used to refer to Australia, New Zealand, and many of the Pacific Islands (note that this doesn’t change the count at all)
Starting in 1995, some scientists claim that New Zealand is the only landmass on a large submerged continent called “Zealandia” (but if we start including submerged continents, that opens up a whole new can of worms; the only reason Zealandia would stand out is because a populated country is above the waterline).
So, what defines a “continent” depends on who you’re talking to: a historian, a geologist, a cartographer, a politician, etc.
Why I Looked It Up
I was researching subcontinent and realized I need to figure this out.