ambient heat 
The combination of air temperature, humidity, air movement and background infrared radiation that surround an object.

ambient heat index

1. A measurement system that creates a single value derived from air temperature and other commonly measured components of ambient heat; usually expressed in degrees C or degrees F. For example: HI, UTCI, WBGT;

2. a single measurement derived as in 1. above

extreme heat

Air temperatures exceed 36°C.

heat stress

1. where the human body is unable to dissipate internal heat to its surroundings by conduction and radiation alone;

2. the direct effects in the body of the above situation

heat strain

The response of the human body to heat stress. For example: sweating, lassitude...

workplace heat

The ambient heat of the workplace.

Terminology/ definitions