Aircraft emergencies are broken down into three categories: Alert 1, Alert 2 and Alert 3. These categories are defined by the Federal Aviation Administration (FAA) to provide standard descriptions and terminology for aircraft emergencies.
Alert 1: Indicates an aircraft is having minor difficulties (i.e., minor oil leak; one engine out on a three- or four-engine commercial aircraft or one engine out on a two-engine general aviation aircraft; fire warning lights; etc.). A safe landing is expected.
Alert 2: Indicates that an aircraft is having major difficulties (i.e., a positive indication of fire on board the aircraft; faulty landing gear; no hydraulic pressure; engine failure on a two-engine large aircraft; etc.). A difficult or crash landing may be expected.
Alert 3: Indicates that an aircraft has crashed on or off the airport, or there is a high probability the aircraft will crash, or the pilot has indicated that the aircraft landing gear will not work and, therefore, the pilot will have to crash land on the airport.
FYI, the aircraft was a Boeing 737 and landed safely.