The term 'major incident' is an emergency services and emergency planning term used to describe an emergency that requires the implementation of special arrangements by one or more of the emergency services ,and generally includes the involvement, either directly or indirectly of large numbers of people. It may also involve other agencies such as Local Authorities, National Health Service, Environment Agency, Military and Voluntary agencies.

The declaration of a major incident by one of the named agencies above allows the release of additional resources in order to respond to and manage the emergency whatever it may be.

A major emergency may require:

• The initial treatment, rescue and transportation of a large number of casualties
• The involvement either directly or indirectly of large numbers of people
• The handling of a large number of enquiries likely to be generated both from the public and the news media, usually made to police
• The need for large scale combined resources of the Police, Fire and Rescue Service and Ambulance Service.
• The mobilisation and organisation of the emergency services and partner organisations, for example, a local authority, to cater for the threat of death, serious injury or homelessness to a large number of people.

For specific health purposes, a major incident may be defined as:

"Any occurrence which presents a serious threat to the health of the community, disruption to the service, or causes (or is likely to cause) such members or types of casualties as to require special arrangements to be implemented by hospitals, ambulance services or health authorities."

The Civil Contingencies Act of 2004 has defined a new definition of Emergency as;

• An event or situation which threatens serious damage to human welfare in a place in the UK
• An event or situation which threatens serious damage to the environment of a place in the UK
• War or terrorism which threatens serious damage to the security of the UK.