To know what child abuse is, one must know what constitutes a child. A child isn't always defined as a person under the age of 18; each state has its own definition.
When it comes to child abuse, state law plays an even bigger role than federal law; learn more about state law here.
For more information, see the Federal Child Abuse Prevention and Treatment Act, 42 U.S.C.A. 5106g.
* This webpage does not serve as legal advice and is intended simply to provide you with basic resources. If you find yourself amidst legal issues, consider engaging legal counsel.