In “Managing The Risks Of Organizational Accidents”, Reason describes organisational accidents as having “multiple causes involving many people operating at different levels of the business”. For individual accidents “a specific person or group is often both the agent and the victim of the accident”. Reason adds that the distinction between the two is hard - an individual accident may turn out to be an organisational accident. As said above the book sees the entire term as a myth because the ‘default search for management failures’ would turn any individual accident into an organisational accident.
I don’t know if Reason actually intended to use his description as a proper definition. One may wonder how useful this is anyhow because one could separate the two only in retrospect - after an investigation. Strictly defined or not, for my understanding the distinction was more that between relatively straightforward cases and more complex cases with sometimes less linear and clear causal chains. The only real value of the term is thus that we are aware of this possible complexity.