Definitions

A tornado near Seymour, Texas.

Tornado
A tornado is defined by the Glossary of Meteorology as "a violently rotating column of air, in contact with the ground, either pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud..."[6] In practice, for a vortex to be classified as a tornado, it must be in contact with both the ground and the cloud base. Scientists have not yet created a complete definition of the word; for example, there is disagreement as to whether separate touchdowns of the same funnel constitute separate tornadoes.[3]

Condensation funnel
A tornado is not necessarily visible; however, the intense low pressure caused by the high wind speeds (see Bernoulli's principle) and rapid rotation (due to cyclostrophic balance) usually causes water vapor in the air to condense into a visible condensation funnel.[4] The tornado is the vortex of wind, not the condensation cloud.
A funnel cloud is a visible condensation funnel with no associated strong winds at the surface. Not all funnel clouds evolve into a tornado. However, many tornadoes are preceded by a funnel cloud. Most tornadoes produce strong winds at the surface while the visible funnel is still above the ground, so it is difficult to discern the difference between a funnel cloud and a tornado from a distance.[3]

Tornado family
Occasionally, a single storm will produce more than one tornado, either simultaneously or in succession. Multiple tornadoes produced by the same storm are referred to as a tornado family. [7]

Tornado outbreak
Occasionally, several tornadoes are spawned from the same large-scale storm system. If there is no break in activity, this is considered a tornado outbreak, although there are various definitions. A period of several successive days with tornado outbreaks in the same general area (spawned by multiple weather systems) is a tornado outbreak sequence, occasionally called an extended tornado outbreak.[6][8][9]

Extracted from Wikipedia