According to the National Weather Service, a derecho is a "widespread, long-lived storm that is associated with a band of rapidly move showers or thunderstorms." Often, according to the NWS, the system will produce damage comparable to that from a tornado, but is typically in one direction along a straight swath. This is called straight-line wind damage.
By the National Weather Service definition, a storm must match certain criteria to be considered a derecho:
1. The wind damage swath must extend more than 240 miles (about 400 kilometers), and also
2. Must include wind gusts of at least 58 mph (93 km/h) or greater along most of its length
Read the complete article at AccuWeather.com: http://www.accuweather.com/en/weather-news/the-derecho-controversy-unders/14278831