Well, it's how much colder humans feel when there's a breeze or wind. I'm sure you've noticed that you feel colder when the wind is blowing on a winter day. When you get out of the wind, you aren't as cold.

The interesting thing is that the wind chill factor doesn't have any effect on stuff like water -- just on people. So, if the wind chill factor is 25 º F (or - 3.89º C) water won't freeze! Cool, huh?

Actually, the formulas for calculating the wind chill factor are pretty complicated... I was going to explain it here, but it's just too icky! You can read more about them at Wind Chill Factors. (This link will take you off of Coolmath!)