What Is a ‘Nautical Mile’?

A minute of arc on the planet Earth is 1 nautical mile. If you were to take the Earth and cut it in half along a line passing from the North Pole through Paris, and then measure the distance of the curve running from the North Pole to the equator on that circle, and then divide that distance by 10,000, you would have the traditional unit for the kilometer as defined in 1791 by the French Academy of Sciences. To travel around the Earth at the equator, you would have to travel (360 * 60) 21,600 nautical miles, 24,857 miles or 40,003 kilometers.

Source: science.howstuffworks.com