Alan Zenreich
I'll take a slightly different approach to the original question's title.

It also can turn into "no longer a hobby" when you stop doing it.

For example, how many of us had other hobbies before woodturning sucked us into the vortex?

Hobbies compete for the finite quantities of free time and brain cells.

For me, the time spent in the shop directly competes with time that could have been spent riding a motorcycle, learning to play mandolin, cooking, photography, videography, etc. There's just not enough time for it all.

Engaging in any one of them feels like cheating on the other hobbies!

Many of our hobbies have fallen by the wayside as new ones usurp the resources and enthusiasm for the old favorites.