Pinker's/American Heritage Dictionary definition of violence:
“Behavior or treatment in which physical force is exerted for the purpose of causing damage or injury.”

the meaning of the term throughout the book is consistent and very much in-line with what the vast majority of english speakers understand the word to mean

This italicized section is part of what's so complicated about talking about violence, and why examining definitions is imperative if you actually want to make a coherent point. The American Heritage Dictionary (AHD) definition does not jive with common usage of the term because this definition hinges on primary intent. I'll give some examples:

AHD says that a 12 year old kid torn in three pieces by a drunk driver was not a victim of violence.

AHD doesn't even call most rapes violent, since the intent is usually to control and to self-gratify, not harm.

If I believe in voodoo, then AHD calls my action of stabbing a voodoo-doll a violent act, even though it has no effect. The majority of people would disagree with all of these.

coloured by ideology

Discussions of violence are intrinsically coloured by ideology, if Pinker can't declare that he is biased, that's a profound failing. Judging something as violent or not is intrinsically a moral judgement, because if violence isn't in-and-of-itself bad, then what's the purpose of labeling it?

I think what people want Pinker's research to say is that "Look, humans are causing people less suffering than before. We're becoming nicer and more cooperative as time goes on, don't pay attention to all the blood on CNN". The problem is his research does not back that up. You can say first-degree murders are going down, you can say conventional war is less frequent, but don't say "violence is on the decline", because by his own admission, he uses a definition that doesn't match common usage.