[ngokevin]

I feel side projects is something you do for fun or on a whim. You just start building immediately on your off-time with not much expectations. Maybe to try out a new technology. It's more of a solo engineering activity. You build it first, and maybe down the line, something comes of it and you do a Show HN in six months.

A business is more broad and deliberate. From the get-go, you need to constantly be talking to many people, have a process for validating the problem, track metrics, plan your time, think about opportunity costs of pursuing different paths, have sales funnels, marketing, scope things out, etc.