Defending yourself (or others) by seeking revenge for something done to you (or others).
Ex. 1- Don: Jack Bauer and Batman just went on a mission to find and apprehend the crazy terrorist who bombed Gotham Los Angeles Plaza.
Bill: Yeah, they both seek defengeance for what happened. As long as the terrorist are out there we aren't safe.
Ex. 2- Whenever a member of a gang sees someone from a rival gang, he's obliged to defengeance himself. He must defend himself from a possible attack (whether immediate or in the future) by seeking vengeance on the rival gang member for all the pain the rival gang has cause him. It's a never ending cycle.