A side hustle is a job generally undertaken to earn extra money over and above your "real job". It is (almost always) providing a service as an individual (for example, house sitting, dog walking) rather than a second job working for an established company. That would be a part-time job, which just means a "regular job" where you work less than the standard forty hour work week. (A part-time job is often filled by a student. Some adults might have more than one part-time job to make ends meet.)
Since many side hustles don't have fixed hours, a person could have two or three or more side hustles.
Sometimes an activity which is your sole source of income, such as day trading financial instruments, is referred to as a side hustle even when it's not a supplement to regular employment.
Side Quest 2024