A numerical measure of asteroid collision hazards similar to the Richter scale for earthquakes. The below was taken wholesale from http://neo.jpl.nasa.gov/torino_scale.html and appears to not be copyrighted.



Events Having No Likely Consequences (White Zone)
Level 0 : The likelihood of a collision is zero, or well below the chance that a random object of the same size will strike the Earth within the next few decades. This designation also applies to any small object that, in the event of a collision, is unlikely to reach the Earth's surface intact.

Events Meriting Careful Monitoring (Green Zone)
Level 1 : The chance of collision is extremely unlikely, about the same as a random object of the same size striking the Earth within the next few decades.

Events Meriting Concern (Yellow Zone)
Level 2 : A somewhat close, but not unusual encounter. Collision is very unlikely.
Level 3 : A close encounter, with 1% or greater chance of a collision capable of causing localized destruction.
Level 4 : A close encounter, with 1% or greater chance of a collision capable of causing regional devastation.

Threatening Events (Orange Zone)
Level 5 : A close encounter, with a significant threat of a collision capable of causing regional devastation.
Level 6 : A close encounter, with a significant threat of a collision capable of causing a global catastrophe.
Level 7 : A close encounter, with an extremely significant threat of a collision capable of causing a global catastrophe.

Certain Collisions (Red Zone)
Level 8 : A collision capable of causing localized destruction. Such events occur somewhere on Earth between once per 50 years and once per 1000 years.
Level 9 : A collision capable of causing regional devastation. Such events occur between once per 1000 years and once per 100,000 years.
Level 10 : A collision capable of causing a global climatic catastrophe. Such events occur once per 100,000 years, or less often.