I agree with you. I would also like to add riding abuse, yanking on the reins, digging the spur in for absolutely no reason, not cooling them out. My father is a professional trainer and a farrier, and he says that it makes him sick to see horses' feet that have never been done or are only done when a rider wants to ride them. They are either too lazy or too cheap to see to the horse's feet on a somewhat regular basis, and then they complain when the horse is lame when they want to ride them.

Have you ever seen what riding in a treeless saddle does to a horse?  A friend of mine went and bought one and after one ride in it her horse had a bump on its back which took about 2 months to go away.  There is a reason for trees in saddles.

Well now there is a legal definition and it is in line with the governing bodies especially FEI

2004 Washington State Supreme Court held in 118 Wn. App. 730, State v. Zawistowski, that Webster's Third New International Dictionary 1621 (1969) definition of "pain" as "a state of physical or mental lack of well-being or physical or mental uneasiness that ranges from mild discomfort or dull distress to acute often unbearable agony."

I agree with all of the above and would add that losing your temper and blaming the horse for your mistakes are hugely abusive, and in fact, shameful bhaviour.

I would also go so far as to say that witnessing abuse and doing nothing is abuse in itself.

Ugggh. I hate what people call abuse. One of my friends who is a self proclaimed horse expert(I tell her to take lessons every chance I get) anyway, when I bought my crop as a tool/ornament for the lesson horses I ride, she told me I was being abusive. So I took it, hit her leg harder then I would a horse. Of course she let out the big "ouch". Five seconds later she was like "wait, that didn't hurt"

I just hate what people define abuse. I also hate some natural horse people. To me "natural" is using the horses(or animals) own instincts/reflex's/personality to train them. So what if I use a saddle with a tree? It's not natural for horses to have people on there back in general, so who cares what saddle I use if it fits correctly, and causes my horse no pain? Oh. I also hate the people who don't know discipline from abuse. A smack(not a hard one) to a horse,dog,child when they do something wrong is discipline. Abuse is repetitive. As I child, I was given a slap on the wrist more then once, I would never call it abuse. People now a days would call that abuse...

I have to admit that I'm not much of a disciplinarian, although I certainly see where one might need to be. I also am very aware of my riding/horse handling style and have tried to buy horses that fit well with my approach.

I would have to mostly agree here with Barb and the first few posters. Abuse is anything really that does not benefit the horse's well being. For example as Nikki noted, lack of proper water/feed/turnout, but also for example as Barb said, seeing/recognising abuse and doing nothing about it.
I agree with Emiry in that there is a distinct line between discipline and abuse. I'm one of those kids that was spanked if I blatantly defied what I knew was right (ie talking back to my mother, being a brat in general.. the likes), and I have no qualms about disciplining children or animals, to an extent. If a horse is blatantly being a jerk to me (biting/nipping, striking, etc) and there's no medical reason behind it, you bet your muckboots I'm going to be stern about it. I once heard when I was younger that Native Americans used to bite their horses ears... so when my POA filly bit me, I bit her back. Now, that's a bit outrageous, but she never bit me again. I don't think disciplining a horse can be considered abuse until it's taken past the point of correction and into excess.

So in a nutshell, I agree with everyone here in their definitions of abuse. Blatant neglect to the well-being of the animal whether direct or indirect, seems to be the definition of abuse.

I agree with all that abuse is lack of care -etc and that abuse can also occur through ignorance.   Also keeping a horse in isolation away from others of it's kind.  I've often seen a horse alone with no chance to interact with others of their kind or even similar species....and I think...since they a social animals, herd animals..how hard this must be for them.  How vulnerable and lonely they must feel. 

Assembly line riding lessons in big facilities where horses - often aged horses- work for 3 hours a day sometimes...carrying beginners...are abusive horsie labour camps as far as I'm concerned...

but using a crop or spurs - appropriately as aids to reinforce when the natural aids have been ignored, or behaviour warrants -(ie a 'hey you listen here..')...and never ever using whip or crop in anger.....EVER...unfortunately people do use these as punishment...because they've had a bad day...and the horse puts a step wrong and whammo....

I think it is abusive to treat your horse like a 'lap dog' though...because you don't do him/her any favours by being too soft and not bearing in mind that a horse is big and powerful, and MUST respect you...or it could be disasterous for horsie and you.