Who uses it: Meteorologists
What it means: The "real temperature" we feel, a combination of heat and humidity -- the summertime equivalent of the windchill factor.
How you can use it: When staying indoors.
The temperature here, right now, is 91 degrees (33 degrees Celsius, for those of you reading in Europe). The good news is that it's not terribly humid, so the heat index is only 94F (34C). I'm not sure that three degrees makes a big difference. Either way, it's too darn hot.
I'm going to the movies... but first, I might take a nap. The drawback to taking a nap is that I will need another shower when I get up, and I've already taken two today.