In its pure form is the instinct to intimidate and use the least contact necessary to drive away a threat to a dog’s self, social status, pups, or anything that the dog feels is important. When a dog bites in defense it is generally with the front of the mouth only and fast with intentions of hopefully driving away the threat while avoiding full contact from that threat. This sums up as, “I’m going to threaten or hurt you, only while you are threatening me or what is very important to me.”
Dogs that show "food aggression" or other resource guarding behaviors are usually in this pure drive. Many dogs will also display this kind of aggression when flight isn't available, such as when restrained at the veterinarians office, toe nails clipped, etc.
Dogs that pursue or otherwise do more than necessary to defend from immediate threat are not in pure defense and are moving into the fight drive category.
To learn more how defense drive relates to other forms of aggression read: Aggression Drives