General opinion is that a side job is one that supports the main income of an individual, often times perceived as something that brings little money to add to what you make from your day job. If Mr X earns better from a side job when compared to the income from his career option, does the side job still qualify to be called a side job?
That depends on, that side job in addition to paying more than the main day job and is really work that can be sustainable and we can participate in the long term. Then we can rethink and consider converting it into daily jobs, but if it has a high income but the work is not regular then I think it can hardly replace the current job.
Indeed. The job called as main job means it is stable (you're employed and its for long term) regardless of the salary. Even you're earning more in your side job it cant be called as main if it is not stable as a source of income. Currently its quite common for people to have a side job, given the inflation for almost everything I think its wise to have a one.