I think the intent has a lot to do with it. If a parent is angry and intends to hurt the child to punish them for whatever they have done that angers the parent, they will do something abusive. I know people who have used the wooden spoon for discipline and it is definitely NOT abuse. In my opinion, a proper way to use something like a spoon on the rear or a spanking is that the parent clearly lays out what the rules of the house are the behavior expectations along with the consequences for disobeying them. That way a child knows if they do something they are not supposed to do, they have received the consequences for that act. Simple cause and effect.
When a parent just gets angry and starts hitting a child because they don't like what they are doing, that is abuse. The child will live in fear of making the parent angry, rather than learning to respect the parent's authority and see the need to obey to avoid punishment.
Obviously, if a parent is doing something to a child that is or could cause injury, that is not OK. Getting a good swat just stings, it doesn't injure, so I think that is acceptable.