When does discipline cross the line and become abuse? My cousin hit her son in the chest with a closed fist as a form of discipline. It was her non-dominant hand and not with full force. Admittedly, her son has behavior issues and didn't seem too fazed by my cousin's actions aside from holding his chest for a moment before running off to play. Is this considered abuse? I don't think any injury was sustained. Does an injury make it abuse?
You'll find many definitions of what constitutes abuse and none are crisp. For example, no one thinks anything of splashing someone in a pool but water torture involves dripping water on a person and that is abusive.
Focusing on physical abuse, a common definition is physical injuries, including bruising, swelling, or broken skin, especially injuries that require medical attention. Yet, this alone is inadequate. For example, many sports can result in injuries and some are severe enough that it requires a doctor's care; yet, we generally don't consider sports to be a form of abuse.
Abuse has no purpose beyond the abuser tearing down the person being abused. There are no lessons being learned to improve the abused person. God gives authority to some for the purpose of building up and not for tearing down. "For this reason I am writing these things while absent, so that when present I need not use severity, in accordance with the authority which the Lord gave me for building up and not for tearing down" (II Corinthians 13:10). At times, there may need to be temporary setbacks in order to reach the betterment of an individual, but an abuser doesn't aim for another person's advancement.
Discipline has, as a purpose, the ultimate improvement of a person. "For they disciplined us for a short time as seemed best to them, but He disciplines us for our good, so that we may share His holiness" (Hebrews 12:10).
When seen this way, then even inaction that allows another person to be harmed can be a form of abuse. This is something our society has lost sight of. "It is for discipline that you endure; God deals with you as with sons; for what son is there whom his father does not discipline? But if you are without discipline, of which all have become partakers, then you are illegitimate children and not sons" (Hebrews 12:7-8).
While you or I would probably not hit a child in the manner that your cousin did, what you described would not be classified as abuse. I would be more inclined to mention to your cousin that while her son needed correction, hitting him in that manner could lead to injuries, and perhaps she should consider other options.