Lightning is defined as any and all forms of visible electrical discharge caused by thunderstorms.
Source: Minnesota Hazard Mitigation Plan
While windstorms and tornadoes are also a significant hazard associated with severe thunderstorms, lightning is probably the most frequent hazard associated with thunderstorms and the hazard that causes the most loss of life. Lightning occurs to balance the difference between positive and negative discharges within a cloud, between two clouds and between the cloud and the ground. For example, a negative charge at the base of the cloud is attracted to a positive charge on the ground. When the difference between the two charges becomes great enough a lightning bolt strikes. The charge is usually strongest on tall buildings, trees and other objects protruding from the surface and consequently such objects are more likely to be struck than lower objects.
While cloud-to-ground lightning poses the greatest threat to people and objects on the ground it actually accounts for only 20% of all lightning strikes. The remaining lightning occurs within the cloud, from cloud to cloud or from the ground to the cloud with in-cloud lightning being the most common.
According to the National Weather Service (NWDS), nationwide, lightning is the number one killer weather phenomena. During a typical year, lightning kills more people than hurricanes, tornadoes, and winter storms combined. NWS-Chanhassen Office estimates that annually, lightning causes two deaths and three injuries in the state.