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?
Seasonality is the nature that we will evaluate to distinguish whether the job is secondary or primary. It is up to you to determine how you will perceive it for yourself based on your personal circumstances.
For example, my main job is selling daily newspapers and a side job is MMO
So it doesn't matter how much income it will bring, because any evaluation criteria is reasonable from that point of view.