If we are taking “harm” to mean “physical or psychological damage or injury,” then a definition of “online harms” could be as follows:
online harms: physical or psychological damage or injury perpetrated in the online/internet space, whether legal or illegal, or within or against relevant platform policy
This is a broad definition and could cover everything from the posting of/engagement with hate speech on social media to the physical harm that arises out of online bullying among children (online bullying leading to physical repercussions in the corporeal space).