I dont have a definition either, but for example the btc lending that takes 10%pa and your bitcoin with 50% LTV is clearly usurious to me. But people on the other side of the game are more greedy and don't want to let go of their BTC. So to each what he deserves.
Discussion
That's the thing. Value is purely subjective and there's no other way to tell, what's right except when supply and demand meet on a voluntary basis. If people don't want to let go of their btc (because of greed or risk aversion), you'll get a high price.