A slap becomes abuse pretty much when the slap happens. Nobody should slap anybody, period. Physical violence shows a lack of mutual respect as a human being and a "serious deficiency in conflict resolution skills." Abused guys, especially big guys, don't get the understanding that abused women do.
I have witnessed domestic violence in a variety of situations, much more than once. And yes, women certainly do attack men, and it seems to me that some of these violent women are unaware of how much pain and even bodily damage they can do. If you're on the receiving end, the thing to do - if you can - is to catch her wrist and with minimal force restrain her until she calms down (to protect yourself, not simply to overpower her into submission.) Be careful, because it can quickly turn in to a wrestling match. (Of course, if your spouse is truly giving you a whompin', like with a weapon, hell ya, you do whatever you have to do to protect yourself!) Then, as dashingscorpio said, call the police and have it legally documented. Then, you and she should see counselors, her for her violence and you for dealing with the question of whether or not you should stay with her.
Personally, I vote for the guy getting the hell out of there. Sure, on TV and in the movies the woman throws the dishes at him and then later they make up and her violence is "just the way some women are", but if the roles were reversed, what advice would people give?
I know of a 100 pound woman who stuck a kitchen steak knife in her 200 pound husband's forearm (where would the knife gone if he hadn't raised his arm?), another who threw a brick at her husband's head (she missed), and a third who attempted to break a big piece of ceramic over the back of her boyfriend's head (he turned in time and deflected it to the floor, and she said, "See what you made me do!") Don't tell me maybe I don't know the full story on these incidents, because I was there, in the middle. Domestic violence gets really ugly really fast.
Physical violence sometimes stops, but it usually only escalates. When a spouse hits someone "back", the other person does not say to him or herself, "Gee, that hurts, physically and emotionally. This must be how my mate feels when I do the hitting. I see the error of my ways." No, they think something more like this: "Oh, ya? You bitch / bastard! Take this!" I figure that if a woman will slap a guy, sooner or later she just might throw a brick at his head.