Definiteness of purpose is key to Outwitting the Devil so says Napoleon Hill. To me, this means focus and action. Focus and action directed at what you desire. Whatever you desire in life is caused by your mentality and the work you put in, based on your mentality. You are what you do, and you can't do anything without thinking first. Thought comes first, then comes action. Once action is based on a definite mindset and goal, life starts to shape itself or become the effect of the mindset/actions.
For example, A person isn't a farmer until he plants and raises crops and does all the other stuff a farmer is known for doing. A football player has to play football. The same way a pile of lumber isn't a wood deck until a mind with the purpose of creating a deck goes to work and uses the lumber for that purpose. You can't be what you don't do.
Definiteness of purpose means doing what it takes to become the person that has the life caused by your mindset/actions. This is what I think Napoleon Hill was getting at, in Outwitting the Devil with his conversation with the "devil". He used the idea of the enemy of mankind to explain that if you don't have a definite purpose, that is aimed at achieving success at something, then you are automatically "drifting" in life. This is what most people do, all the while wondering why things aren't going their way. You have to live with a definiteness of purpose to cause the life you desire. Life is cause and effect and all mind.