Chris posted an interesting question.
Hope it's ok I started a thread a for it. Stuck laying in bed and using the blogger app for my phone.
For me it's a project that gets put a side for a non specific length of time that you just stop working on for what ever reason. Except those that are put aside because it's not it's rotation turn that is. The youngest/shortest time set aside for me is my Greek Mandala. It's been mainly untouched since Nov of 2011. Mainly to finish up some smaller UFO's and things in life in general.
But I think one can say any project that isn't finished is a UFO.
What does everyone consider a UFO? Do you let it sit for a certain amount of time? Does a UFO turn into a WIP the moment you pick it up? Is it both? Or does it just need to feel like a UFO?