I have no cites for the below. All answers are based on my personal experiences both as a parent and as a child, and on my obervations of other parents and children. I assure you, I have given a great deal of thought to the subject over the years.
The key, as LilShieste pointed out, is punishing the behavior. As a parent, you’re trying to indtroduce the concept of negative consequencens for certain actions. Pain and humiliation, as grimpixie calls spankings, are certainly negative consequences. But, all punishments are humiliating in some way or another to a child, in my opinion. Similarly, all punishments cause pain of one kind or another, be it physical or emotional. If, for example, you take away the childs favorite toy, it’s emotionally painful. If you make them stand in the corner, it’s humiliating. The two are necessary to effective punishment, but must be carefully moderated.
Punishment itself should be tempered with reason, as the child becomes old enough to understand reason. Punishment (be it a smack or a timeout) should never be doled out without first discussing the reasoning behind it. The children learn that, not only do their actions have consequences, but why those they have those consequences.
I think parents who think before they smack are (generally) not being abusive. The parent who hits their child in anger, however, is worrying.
Grimpixie:
A smack is not the top a punishment scale. It depends on the child, the situation, etc. My children would much rather get a smack than sent to bed early, for example. Plus, as children get older, physical pain is generally less effective and creates hostility between the child and the parent, which is certainly not the goal. As the child matures, older forms of punishment are retired and different strategies employed. Instead of a smack, now there’s no more Nintendo.
And finally, the key to making any punishment most effective as a teaching aid is love. If the only time you pay attention to a child is to punish him, you are doing a disservice to your child.
For me, the key is watching their behavior with others. My children are happy, they enjoy my company and the company of others, they seem well adjusted, they play nicely with others, they’re pretty sure of themselves, etc. So, I think I’m doing a pretty good job. I’ve seen other children who are sullen, generally unhappy, liking making other kids cry, etc; and I’ve seen the way their fathers behave with them. I tend to think those parents are not doing such a good job.
But, we only find out in the long run.
The short answer to the OP: A spanking, in and of itself, is not abusive. However, like so many other things, it all depends on circumstances. YMMV.