Under the Stafford Act, the President can designate an incident as:

An emergency, or

A major disaster.

In certain circumstances, the President may declare an “emergency” unilaterally, but may only declare a “major disaster” at the request of a Governor or tribal Chief Executive who certifies the State or tribal government and affected local governments are overwhelmed.

Emergency:

Any occasion or instance for which, in the determination of the President, Federal assistance is needed to supplement State, tribal, and local efforts and capabilities to save lives and to protect property and public health and safety, or lessen or avert the threat of a catastrophe in any part of the United States. A variety of incidents may qualify as emergencies. The Federal assistance available for emergencies is more limited than that which is available for a major disaster.

Major Disaster:

Any natural catastrophe (including any hurricane, tornado, storm, high water, wind-driven water, tidal wave, tsunami, earthquake, volcanic eruption, landslide, mudslide, snowstorm, or drought), or, regardless of cause, any fire, flood, or explosion, in any part of the United States, which in the determination of the President causes damage of sufficient severity and magnitude to warrant major disaster assistance under this chapter to supplement the efforts and available resources of States, tribal governments, local governments, and disaster relief organizations in alleviating the damage, loss, hardship, or suffering caused thereby.

Major disasters may be caused by such natural events as floods, hurricanes, and earthquakes. Disasters may include fires, floods, or explosions that the President feels are of sufficient magnitude to warrant Federal assistance. Although the types of incidents that may qualify as a major disaster are limited, the Federal assistance available for major disasters is broader than that available for emergencies.