Thunder is the loud, rumbling sound you hear during a storm, caused by lightning suddenly heating the air. When a lightning bolt flashes through the sky, it creates an incredibly hot channel (hotter than the surface of the sun) that makes the surrounding air expand faster than the speed of sound. This creates a powerful shock wave that reaches your ears as thunder.

The sound can vary dramatically: sometimes it's a sharp crack when lightning strikes nearby, and other times it's a long, rolling rumble when lightning is farther away. You can estimate how far away lightning is by counting the seconds between the flash and the thunder, then dividing by five to get the distance in miles. If you count five seconds, the lightning struck about a mile away.

The word also describes any loud, booming sound that resembles a thunderstorm. Horses' hooves might thunder across a field, or a herd of elephants might thunder through the savanna. When an audience applauds wildly, their clapping might sound like thunderous applause. A coach might thunder commands at practice, meaning she speaks in a powerful, commanding voice that everyone can hear.