Defensiveness is anything we use to move away from responsibility for something that our spouse believes that we have done or not done. Defensiveness like it implies, means that we perceive what our spouse is saying about or to us as an attack. In response (whether it was done in a respectful way or in a way that was truly an attack) we employ defensiveness to keep from receiving responsibility.
Defensiveness can come in many forms. Stating clearly that you did not do something and listing the reasons may be defensive. Counter accusing your spouse with something that they did not do as well is defensive. A spouse might say, “I noticed that the grass is not mowed and is really getting tall. Would you mind mowing the grass today or tomorrow?” And the defensive response might be, “I would have mowed the grass sooner but you said you would start putting your shoelaces in the closet instead of leaving them all over the house.”