A side hustle is just extra work you do in addition to your normal job. A side job or side gig, in other words.
The phrase “side hustle” doesn’t have a bad meaning the way like the word “hustle” alone sometimes does. It’s more like hustle in the sense of “get a move on and work hard”. (Like your gym teacher might have yelled.)
It can be work you do a few hours a week, no set number of hours, or even a lot of hours if you are full of fire.
Usually a side hustle is done as a freelancer or independent contractor/entrepreneur (aka working for yourself), so those are the kinds of things we’ll focus on here.
But working part time for an employer can also be called a side hustle if you’re doing it on top of your regular job.