Well technically I don't know of any weapon system that is strictly geared toward defense or offense.
The
@Revoltingest stated this which is accurate and inaccurate.
Let's look at his example of Offensive.
True that they can not directly defend against ballistic missiles, just as a SSNB or FBN submarines can not directly defend us against first strikes. Both of them can be seen as either a offensive (first strike) or defensive(MAD). When it comes to amphibious assault ships and their supporting forces can be seen as offensive to land and support capturing of territory or defensive in the role of ensuring that our allies can not be overrun or as a "big stick". Remember the best defense is an good offense.
Now to his defensive examples.
Directed energy weapons can be seen as an offensive or defensive weapon. No they can not attack but they still can be an offensive weapon. Depends on the role they are used in. As an offensive weapon they could insure we could attack a nation and have no fear of retaliation from ballistic missiles. Defensive to insure that we will not be attacked first.
So basically any military weapon can be categorized as defensive or offensive, it just depends the role it is used for.
Whoever changed the War Department to the Department of Defense did the public a great disservice. The military is a war machine, just because Dept of Defense sounds more benign that the Dept of War does not change the role of the military.