Abuse is multi-faceted really. You have the non consent stuff. People whose boundaries are pushed to a point they feel uncomfortable.
Doms having scenes with subs who are clearly vulnerable or mentally unwell. They make consent but are in no fit state too. Similarly if a Sub has been drinking or has taken drugs.....going near them would be abusive.
Going beyond any agreed boundary, not respecting safe words, not giving a sub means to say stop if she was gagged (I have used keys in my hand to drop).
Photographing or videoing either without a persons knowledge, without their consent, or when they are in no fit state to agree.
Not being listened to and properly heard, being criticised a lot, deprivation of food/use of toilet etc, involving third parties without consent. all are abusive.
Plus all the other abuse that happens in normal relationships.