the.com/heat index

means It's the 'feels like' temperature that combines actual air temperature with humidity, because sweat can't evaporate to cool you when the air's already saturated.

from A 20th-century meteorological coinage, not folklore — built from 'heat' plus 'index' in its mathematical sense of a calculated number derived from a formula, first developed by researchers modeling how humidity sabotages the body's evaporative cooling. The underlying science (dew point, evaporation, skin temperature) dates back further, but the named, standardized index itself is a mid-to-late 1900s invention, created so weather reports could give people one honest number instead of two misleading ones.