What is the visible electrical discharge during a thunderstorm?

The visible electrical discharge during a thunderstorm is lightning. It is a powerful burst of electricity that occurs when separated electrical charges in a cloud, between clouds, or between a cloud and the ground suddenly equalize.

Thunderstorm clouds contain turbulent mixtures of ice crystals, supercooled droplets, and soft hail called graupel. Collisions among these particles help separate positive and negative charge. When the electric field becomes strong enough, a conductive path forms through the air, producing the bright channel seen as a flash.

A lightning channel can heat the surrounding air to roughly 30,000 kelvins, causing that air to expand explosively. The resulting pressure wave is thunder. Because light travels much faster than sound, observers see the flash before hearing the rumble; counting seconds between them gives a rough estimate of distance.

Lightning is not the same thing as thunder. Lightning is the electrical event, while thunder is its sound. Hail and rain may accompany both, but neither is the visible discharge itself. Lightning can also occur within a cloud or far beyond the area where heavy rain is falling.