It’s a major raise in body temperature. When you have a high temperature we usually call it one of two things, febrile or hyperthermic.
Febrility is an inside job, you get fevers when there’s inflammation/infection/etc goin on in the body, and your brain’s hypothalamus regulates your temperature from within.
Hyperthermia is an outside job. Our environment is heating us up, and with severe hyperthermia, or heat stroke, we’re above 104 Fahrenheit or 40 degrees celsius, and we can’t cool ourselves off on our own by sweating, breathing, and all that good stuff.
Heat stroke is no joke and can land a person in respiratory failure, seizures or in a coma.