A lightning strike is an electric discharge on a massive scale between the atmosphere and an earth-bound object. They mostly originate on the ground and terminate in the thundercloud, called ground to cloud (GC) lightning. Upward propagating lightning is initiated from a tall grounded object and reaches into the clouds. About 25% of all lightning events worldwide are ground to cloud. The bulk of lightning events are intracloud (IC) or cloud to cloud (CC), where discharges only occur high in the atmosphere. Some lightning strikes go from cloud to ground (CG).
A single lightning event is a "flash", which is a complex, multi-stage process, some parts of which are not fully understood. Most ground to cloud flashes only "strike" one physical location, referred to as a "termination". The primary conducting channel, the bright coursing light that may be seen and is called a "strike", is only about one inch in diameter, but because of its extreme brilliance, it often looks much larger to the human eye and in photographs. Lightning discharges are typically miles long, but certain types of horizontal discharges can be upwards of tens of miles in length. The entire flash lasts only a fraction of a second. Most of the early formative and propagation stages are much dimmer and not visible to the human eye.
Thunder is always produced by lightning, but very distant lightning may be seen but not heard. Lightning cannot happen in a vacuum devoid of ions, nor can thunder occur without molecules to vibrate.