A defensive driver is one who will do everything possible to avoid a collision.
That means that they are 100% on the task of driving and are not distracted by stereos, cell phones, passenger conversations, noisy children, the scenery outside, or anything. That means that they are mentally prepared for that task and are well rested, not ill, nor impaired by drugs, alcohol, medications or anything else. It means that there vehicle is well maintained and kept in a safe condition, like with properly inflated tires, clean windshields and wipers, reliable engine and suspension.
It means that they not only watch their own behavior, they are watching out for the other driver's behavior. The approach intersections with caution, regardless of green or red lights. They anticipate light changes, lane changes and vehicles and pedestrians entering from either side. A defensive driver is watching the traffic flow in front by several hundred yards, behind by nearly as far and to each side for a couple of lanes. Their head are on a swivel with a regualar pattern of looking ahead, to the rear view mirror, ahead, to a side mirror, ahead, to the other side mirror, ahead, to the dashboard instuments, ahead and so on.
A defensive driver is one that who knows that nothing is more important, during the drive that the complete control of his vehicle, and is aware of EVERYTHING possible that can effect his vehicle.
Certified Instructor, National Safety Council Defensive Driving Course 4