There isn't an unique way (or standard) to define a continent. Subdivisions or unions are made after the first big definition, which is Europe, Asia, America, Africa, Oceania and Antarctica.
Example of subdivision: NA and SA
Example of union: Eurasia (not currently used, or more related to geology matters)
Or at least that's the way i've learned it at some point of my life. School is way back for me lol