What Defines Child Abuse?

There are certain parameters that are required to be followed to describe an act as an act of child abuse and it can vary from country to country. But there are some parameters that are universal to the fact and can be described to define the act.
      
The psychological or physical maltreatment of a child by an adult can be called as an act of child abuse. The adult can be anybody; he could be one of the parents or any other guardian or adult. The term puts emphasis on some wrong acts; you can also add the act of negligence towards that child as an act of child abuse.
     
But these days the sexual abuse of a child is the only thing that is popular description for the term child abuse.

Well to start with the obvious it means to abuse a child, so to define abuse it means to treat in harmful, injurious or offensive way as well as to speak harshly, or insultingly to. Generally it is defined as to mentally, physically or sexually assault someone.
Vicky is right in saying that it varies by country for in certain cultures to whip your child is just what is traditional, but here in the United States it is abuse because it fits the terms I listed above. I hope that my answer has helped you.