You're confusing prime ministers in parliamentary democracies, e.g. the UK with presidents [1].
[1] https://www.britannica.com/story/whats-the-difference-betwee...
True, there's a minority and majority.
> journalists tend to impose the term on non-US presidential systems
Didn't President Yoon call them the opposition? Or is that a liberal translation?
In a parliamentary democracy, the governing coalition and opposition are clearly delineated. In a presidential system, a legislature controlled by a party different from the president tends to be referred to as an opposition legislature, e.g. the House is currently in opposition to Biden.