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?
Yes. If you are a Cop then you put up a business in your town which gives you bigger amount of income compare to your salary, then I think it is still consider as a side job. For me, the main reason for having a main job is its security. You are secured to have an income even if your side job fails to do that.