The phrase "heat index" refers to the level of discomfort a person feels due to the combination of heat and humidity. It helps us understand how hot it actually feels, rather than just relying on the temperature alone. Full definition
Don't be jealous because I was sweating bullets when I wore it for photos and not to mention it has almost been 100 degrees this week with heat index. (fortheloveofglitter.com)
Heat: When there is hot sun, high humidity and higher heat indexes, the threat of overheating is real. (michigananimalhospital.com)
100 degree heat indexes and pregnancy are not a good combination. (aleciamariana.com)