The phrase comes from the Children Act 1989. Section 31(9) defines harm as ill-treatment or the impairment of health or development, including impairment from seeing or hearing the ill-treatment of someone else. Development covers physical, intellectual, emotional, social and behavioural development. Where the question turns on health or development, section 31(10) says the child should be compared with what could reasonably be expected of a similar child.
There is no list of injuries or events that count as "significant". It is a judgement about the seriousness and impact of the harm, and it can come from a single event or from a pattern over time. Working Together 2026 says it can take many forms, including abuse, neglect, domestic abuse, exploitation, online abuse and the influences of extremism, and that it should be assessed through an anti-racist, anti-discriminatory and culturally aware framework (paragraphs 242 and 246).
For a school, the practical test is whether to refer. KCSIE 2026 says that where a child is suffering, or likely to suffer, harm, a referral to children's social care should be made immediately (paragraph 65). The DSL doesn't have to prove significant harm; the local authority decides whether the threshold for section 47 is met.
Don't confuse it with the harm threshold for allegations about staff, which is a different test in Part four of KCSIE.