A thunderbolt or lightning bolt is a symbolic representation of lightning. It appears variously in history, literature, and in contemporary warnings of electricity. Thunderbolts may appear naturally among the estimated 8.6 million lightning strikes per day or not: heat lightning is an electrical discharge in the atmosphere without an accompanying sound, and a Tesla coil produces an artificial "lightning"-like electrical discharge with an accompanying clap. The term "thunderbolt" adds the notion of a loud thunderclap accompanying a lightning flash, while the term "lightning bolt" — which refers directly to the electrical discharge — does not.