This is reserved mostly toward competitors of "equal" or undetermined status. A dog in fight drive will perceive his target as a threat to something important, but will also go out of his way to continue to inflict injuries with an absolute purpose in mind such as total submission, surrender, or retreat. It is possible for a dog to kill a perceived competitor while in this state. This dog can also toggle back to pure defense drive and then to flight if losing a "fight". In some cases, depending on many genetic and situational factors, a dog may move into a pure prey drive and go through the motions of killing as if the competitor is now prey.
Fight drive may manifest as “territorial aggression” (dogs attack competitors on their territory), during “fear aggression” (I am going to get you, before you get me), and also during "dominance" disputes between dogs or even dogs and people that are perceived competitors for social status and the benefits that go with it (leadership and first right to resources). The term natural aggression is sometimes used in the working dog world to describe a dog that shows pronounced fight drive. Natural aggression is a term that has been derived to be in contrast with aggressive looking behavior triggered by barrier frustration. Although they may look similar, natural aggression is triggered by more primitive instincts to fight a competitor, while barrier frustration has been selectively bred into some breeds of dogs to bark and act aggressively when frustrated or restrained physically from getting to anything desired.
To learn more how fight drive relates to other forms of aggression, read: Aggressive Drives