By definition, thunder is the loud noise that occurs when atmospheric gases are suddenly heated by a discharge of lightning.
Thunder is made up of a series of sound vibrations caused by lightning strikes.
You always hear thunder after the flash of lightning and with good reason. Lightning travels at the speed of light, an estimated 186,000 miles per second or one mile in 5.3 millionths of a second. Sound waves, on the other hand, travel at a snail's pace when compared with the speed of light, only 1,088 feet per second or one mile in about five seconds. To figure out the distance of a lightning strike from where you are, you merely have to count the seconds between when you see the flash and when you hear the thunder. This will give you a fairly accurate estimate of the number of miles between you and the lightning strike you just witnessed.