Side hustles are the business du jour (even in my best-selling book!), but people vary on what, exactly, a side hustle is. I've heard Uber drivers, online retailers and other business people say they are doing a side hustle.

Chris Guillebeau's new book Side Hustle attempts to break it down based on, in his words, a life composed of side hustles. Last night, we connected in Detroit for his 100-city book tour, and he shared an excellent definition of side hustle:

A side hustle is not a part-time job. A side hustle is not the gig economy. It is an asset that works for you.

Picking up a few extra hours at the factory or at the coffeehouse is not a side hustle. Jumping on TaskRabbit or Uber when you feel like it isn't a side hustle, either.

The problem? Someone else can pull the plug. Uber's former co-founder Travis Kalanick openly said he planned on replacing drivers with self-driving cars. Other gig economy businesses have literally folded overnight. If the intention of a side hustle is to create financial independence, then working within the gig economy is a walk in the exact opposite direction.

I wrote about this in a recent piece for SAGE Business:

And gig-economy workers are at the mercy of the startups, lacking the benefits and legal protections of permanent employees, even those who can be terminated for any reason. [The New Yorker's Jia] Tolentino tells the story of a pregnant Lyft driver who felt so compelled to keep driving, perhaps because of the low pay rate, that she picked up a fare even as she was driving herself to the hospital, in labor, to give birth.

It is an asset that works for you; you don't work for it. Cuddlr was a side hustle to my journalism. No one could pull the plug but us - and no one else could make it a success, either. We cashed the check when it was acquired.  It took care of us.

Guillebeau emphasizes that a side hustle isn't meant to be your primary income. As I've talked about in The Ultimate Bite-Sized Entrepreneur, the pressure of profit can crush your most brilliant ideas before you get out the gate. Worse, if it fails, you can't take of your bills. Remember that your side hustle should add, not subtract from your stability.