1911 Encyclopædia Britannica/Thunder

THUNDER, the noise which accompanies or follows a flash of lightning, due to the disturbance of air by a discharge of electricity (see Lightning; Atmospheric Electricity and Meteorology). The Old English word is þunor, also the name of the Scandinavian god Thor (q.v.), which is cognate with Dutch dander, German Donner. The root is than,- Indo-European tan-, cf. Latin tonare, tonitru. This root is apparently another form of stan-, as in Skr. stan, to sound, thunder, Gr. στένειν, to groan, Eng. “stun.”