The NHS definition of a Major Incident
“Any occurrence that presents serious threat to the health of the community, disruption to the service or causes (or is likely to cause) such numbers or types of casualties as to require special arrangements to be implemented by hospitals, ambulance trusts or primary care organisations”
NHS Emergency Planning Guidance 2005
Meaning of “emergency”
(1) In this Part “emergency” means—
(a) an event or situation which threatens serious damage to human welfare in a place in the United Kingdom,
(b) an event or situation which threatens serious damage to the environment of a place in the United Kingdom, or
(c) war, or terrorism, which threatens serious damage to the security of the United Kingdom