Also used as a derogatory term for any argument in which the proponent of an issue is attempting to win the debate via distraction or bizarre changing of subject. Usually the change of subject is followed by attempts to draw parallels between the original topic and the new one.
In the following example, Mr. A. uses a Chewbacca Defense (note the indefinite article):
A: I believe we should have a national health care system!
B: But what about the great public burden?
A: It's part of the Separation of Church and State!
B: What? That doesn't make sense at all. That's a Chewbacca Defense!
This method of rhetoric is often employed by politicians, lawyers, and others who are forced to work from untenable positions.