Adult Social Care Glossary, v 0.6

(7) In this regulation-- 'notifiable safety incident' means any unintended or unexpected incident that occurred in respect of a service user during the provision of a regulated activity that, in the reasonable opinion of a health care professional, could result in, or appears to have resulted in-- (a) the death of the service user, where the death relates directly to the incident rather than to the natural course of the service user's illness or underlying condition, or (b) severe harm, moderate harm or prolonged psychological harm to the service user;

Any unexpected event that causes any sort of harm to a person while they are receiving treatment or care. If this happens, the law says that care providers have to tell the person (or someone acting on their behalf) what has happened, apologise for it, explain what the effects might be, and offer to help fix the situation.