Jay Wiseman suggested a definition of hurt vs harm that has been kicking around local BDSM circles for many years. I agree with it wholeheartedly. It has been shared with medical doctors and mental health professionals, who recognize and have helped refine the definitions and distinctions between healthy, consensual BDSM play and abuse.

To hurt someone means to cause them temporary pain which goes away after a reasonably short duration without any outside intervention.

In other words, the bruise or redness fades, the bleeding stops, the tears dry up, the marks and effects of the hurt go away and the person returns to their former state of physical and emotional health, whatever that state might be, with the help of no more than a Band-Aid and some Neosporin if necessary.

To harm someone means to cause them damage that seriously needs the intervention of an outside professional to heal, ie, you need more than a Band-Aid and Betadine. A doctor needs to tend your injury, or an outside counsellor is needed to put your head back together.

Psychological "damage" can be harder to quantify, but it can certainly be inflicted in the boundaries of a relationship - whether that relationship is SM or not.

Some borderline activities between hurt and harm include permanent marks, scars, tattoos, brands, etc, which are of long term duration and which do not go away.

Generally, if the person who has been permanently marked actively consented to the mark, it isn't considered harm. If the person wasn't expecting to be permanently marked or scarred and it's done to them anyways, it's generally considered to be harm.

It is not always possible for a dom to know for sure whether a punishment will hurt or harm. It should not be considered abuse if your dom accidentally hit a bad button and triggered an emotional trauma point that harmed you. If it's consistent and he doesn't provide aftercare, then you should be concerned.

You are responsible in large measure for knowing what will hurt or harm you, knowing the difference, knowing it is different for different people, and communicating your needs. Your dominant is not psychic, and needs your clear communication to make scening safe and healthy for both of you.