Humidex is the same thing as heat index. I think I like the word humidex better, actually. Heat index implies that something is actually that hot when it isn't.
The actual temperatures this summer (here) haven't been above 105 F /40.6C yet. The humidity, though, after the drought eased up, makes it pretty steamy outside.
Right now at 8:30 am, the humidity is at 85% while the temp is 82F. It grimly amuses me when the humidity number is higher than the actual temperature. The heat index/ humidex is 94F already. It's going to be a bad one.