torinoscale
Used to categorize the threat of asteroids, the Torino Scale is similar to the familiar "Richter Scale" of earthquake measurement. It is named after the city in Italy in which it was adopted during a workshop in June, 1999. The scale uses numbers and colors to indicate risk of collision, all based on complicated analysis of an asteroid's path and calculations of how that path might change as it's affected by gravity from Earth and other bodies.
 
Overall risk Specific categories
Events having no likely consequences 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 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 2. A somewhat close, but not unusual encounter. Collision is very unlikely. 

3. A close encounter, with 1% or greater chance of a collision capable of causing localized destruction. 

4. A close encounter, with 1% or greater chance of a collision capable of causing regional devastation.

Threatening events 5. A close encounter, with a significant threat of a collision capable of causing regional devastation. 

6. A close encounter, with a significant threat of a collision capable of causing a global catastrophe. 

7. A close encounter, with an extremely significant threat of a collision capable of causing a global catastrophe.

Certain collisions 8. A collision capable of causing localized destruction. Such events occur somewhere on Earth between once per 50 years and once per 1000 years. 

9. A collision capable of causing regional devastation. Such events occur between once per 1000 years and once per 100,000 years. 

10. A collision capable of causing a global climatic catastrophe. Such events occur once per 100,000 years, or less often.

explorezone.com . Source: Richard Binzel, MIT