Source:https://blogs.lse.ac.uk/businessreview/2019/04/20/preventing-online-harm/
I think the definition of online harms is online users who encounter harmful online behaviour or content while browsing. This is especially for underage users who are using normative issues. The reason is that illegal and offending content and activities are prevalent online. For example, hate speech, bullying speech, sharing illegal material and incitement to violence.