Here is one example of a legal definition of child abuse (from the State of California):
Definitions of Child Abuse and Neglect
To better understand this issue and to view it across States, see the Definitions of Child Abuse and Neglect: Summary of State Laws (PDF - 442 KB) publication.
Physical Abuse
Citation: Penal Code §§ 11165.6; 11165.3
Child abuse or neglect includes:
Physical injury inflicted by other than accidental means upon a child by another person
Willful harming or injury of the child or the endangering of the person or health of the child
Unlawful corporal punishment or injury
Willful harming or injuring of a child or the endangering of the person or health of a child means a situation in which any person willfully causes or permits any child to suffer, or inflicts thereon, unjustifiable physical pain or mental suffering, or having the care or custody of any child, willfully causes or permits the person or health of the child to be placed in a situation in which his or her person or health is endangered.