According to the Emergency Management and Civil Protection Act of 2006, an emergency is defined as:
"a situation or impending situation that constitutes a danger of major proportions that could result in serious harm to persons or substantial damage to property and that is caused by forces of nature, a disease or other health risk, an accident or an act whether intentional or otherwise"
There are three main categories of emergencies:
Natural e.g. tornadoes, floods, snow and ice storms, disease outbreaks
Human caused; Accidents e.g. train derailments, power outages or Hostile Acts e.g. chemical and biological threats, explosions
Technological e.g. failed computer, telecommunications or other information technology