A multi-casualty incident (MCI) is essentially any medical emergency with more than one patient, although it is generally defined as at least three. In emergency medical services, the term multi-casualty incident is used to trigger a change in the way patients are handled to more efficiently allocate resources to the most patients possible. The change in how patients are handled might be jurisdictional - just one city or county - or it could be statewide or regional with multiple states involved.
Depending on the area where one lives, there are different definitions of what qualifies as a multi-casualty incident. Having more than one patient isn't enough by itself to trigger a different procedure. In general, each ambulance is capable of handling one critical patient, and some can handle two relatively severe cases at a time. Likewise, three or four very minor patients might be transported to a hospital by one ambulance.
To qualify as a multi-casualty incident, the number and severity of patients has to overwhelm available resources. Very rarely will two patients ever be declared a multi-casualty incident. However in a small town or rural area, two critical patients plus another minor injury patient could be enough to trigger local emergency services to declare the MCI. In a more urban area, it might take five or more patients before declaring a multi-casualty incident.
The declaration of a multi-casualty incident means patients are handled in order of severity rather than first come, first served. The process of sorting by severity is called triage. Triage categories differ by area but are usually color-coded. They include green for minor injuries or illnesses, red for critical patients, yellow for those who fall in between and black for patients who are not viable (dead or almost dead).
By definition, a disaster is a multi-casualty incident, but not all multi-casualty incidents are disasters. Disasters are much larger incidents involving several jurisdictions and sometimes several states. Hurricanes and other severe weather conditions are often responsible for disasters, but most crashes and multiple victim violence don't rise to the level of disaster. An obvious exception to that would be the terrorist attacks on Sept. 11, 2001.