I think Stealthhammer has the right idea: it varies from rider to rider depending on their abilities. And I would also add age; I'm a lot more cautious than I was 20 years ago when I got my first rigid bike.

My own personal definition is based on the price of failure. When I approach a trail I have never ridden I'm more cautious than on trails I'm familiar with. I ask myself what sort of injuries and down time am I looking at if I make a mistake? A flat, buff, 2ft wide singletrack can be extremely technical for me if there is a sheer drop on one side (google images Portal Trail Moab). While a nasty singletrack rock garden that is bordered by soft brush might not be that technical (and I hate rock gardens, every rock seems to have broken bone written all over it).

I like to push myself, just not as far as I used to. Our bodies just don't heal from broke bones and soft tissue injuries as fast or as well as when we were young.