I agree that the definition differs between people.

I never leave my child to cry. To me crying is where the child is crying out for someone or something, or is distressed. It is different to whinging which is more like a groan especially if they have their eyes closed and are obviously trying to sleep. CC does work but it is thought to have negative long-term effects with regards to the childs ability to express emotion, their confidence, and their bond to their parents. The more you respond to your child the more resilient they are for those times when you can't respond (eg. if you are in the car and they start crying while you are driving).