I'll call it active and totally optional participation in an area of interest unrelated to one's means of subsistence or social obligations. I'd further go one to describe 'active' in this context to imply the acquisition and application of knowledge and skills in a non-competitive context. (Yes, there may be an element of remuneration or competitiveness associated with some hobbies, but that should be no more than coincidental to the primary objective of indulging your mind in an activity of interest without any external pressure with respect to execution or results.) In short, it's a healthy brain vacation.
Of course there are going to be ways to torpedo any definition you try to put to it... (for instance, once I'm into THIS hobby I can no longer consider water changes, feeding, etc. as optional! But then I don't have to be in the hobby at all...)