Though the line is hugely blurred with projects, I think a hobby should be something that is almost entirely "just for fun". You could have a goal in mind, but that’s not why you’re doing it. You’re doing it because you enjoy doing it.
(Mind you, projects can absolutely be "pointless' and just for fun projects. But in my mind, the emphasis on a project is creating something specific or accomplishing something specific.)
I really enjoyed John Jacobsen’s "In Defense of Hobbies":
I think it’s time to rehabilitate the word "hobby." Unlike professions, hobbies can wax and wane without shame.
My art and working on my website are definitely hobbies. I always end up returning to them because I love doing them, but I do it (mostly) without any sort of schedule or deadline.