Defensive driving is a set of techniques that can be used to forsee potentially dangerous situations when driving, and to modify the driving to avoid those situations.
The defensive driver is aware of what is happening around the vehicle far enough ahead to allow time to adjust and compensate for any possible dangerous situation.
Before starting a vehicle a defensive driver checks the vehicle and the road and weather conditions and makes mental allowances that will be applied when driving, to reduce the possibility of an accident.
When driving the defensive driver always assumes that other drivers may make mistakes, and is prepared for that to happen. Defensive drivers are constantly aware of situations that could develop into something dangerous, and adjust their position, speed and direction in order to be able to maneuver in safety if those situations do develop.
Much like aircraft pilots, defensive drivers are constantly on the look out for hazards. They don't only look ahead far enough to react, but also are aware of vehicles approaching from behind or passing, and they look around before changing speed or direction.
Vehicle and road safety is not an accident! I recommend that all drivers take defensive driving training, practise the defensive driving techniques whenever they are driving, and periodically have a check drive with a defensive driving instructor.