What’s Assault?

Here’s a very controversial matter I’ll speak about: spanking. But I’m going to make it short. I just want an answer to this question.

A few days ago, I overheard what I think was a crime show my grandfather was watching. The episode was featuring a woman who’d escaped from a very abusive relationship, and she described some incidents of the abuse. One of them was when her husband had told her to lean over the hood of his/their car and proceeded to spank her with his belt. She outright said he was hitting on her behind, so it was what’s considered a spanking. She described the pain as being so terrible, she was hoping she would somehow melt into the engine of the car (or in other words, die right there on the spot) to avoid having to feel any more. She said every time she didn’t listen to her husband, he would do this to her.

This woman is an adult, so that is classified as abuse. But if she were a child, that would be discipline. Why? That’s my question. Why is this “abuse” when done to an adult, but “discipline” when done to a child?